VMware ESXi and vSphere Cluster Management

What Is a Virtual Machine? Hosts, Guests, and Hypervisors Explained

Learn what virtual machines are, how hosts, guests, virtual hardware, and hypervisors work, and how to run Linux in VMware Player on Windows.

A virtual machine (VM) is a software-based computer environment. It can boot an operating system, provide virtual hardware to that operating system, and run applications much like a physical computer.

A VM is more than an ordinary application window. The virtualization software creates a complete machine environment for a guest operating system. For example, Linux can run inside a VM while Windows continues running on the physical computer.

In this lesson, the example is VMware Player installed on a Windows computer with Linux running inside a VM. The same host-versus-guest concepts apply when the host computer runs Linux instead.

Virtual Machine Terms at a Glance

• Physical machine: The actual computer and its installed hardware.
• Host system: The physical computer that supplies resources to one or more VMs.
• Host operating system: The main operating system installed on the physical computer.
• Guest operating system: The operating system installed and run inside a VM.
• Hypervisor: Software or firmware that creates and operates VMs while coordinating hardware access.
• Virtual hardware: The CPU, memory, disks, network adapters, and other devices exposed to a guest OS.

What Is a Virtual Machine?

A VM is a software-defined computer. Its processor, memory, storage, and other devices are represented and managed by virtualization software instead of being provided as a separate physical computer.

When a VM starts, the guest OS sees a set of computer devices. It can boot from installation media, create files on a virtual disk, use a virtual network adapter, and launch programs. The guest OS is installed and administered much like an operating system on a physical machine.

The guest does not normally control the host computer's hardware directly. Instead, the hypervisor presents virtual devices and translates or coordinates requests for the physical hardware available on the host.

Virtual Hardware Inside a VM

Before starting a VM, you configure the virtual hardware that the guest will see. This hardware is backed by, or connected to, resources on the physical host.

Virtual CPU and Memory

A virtual CPU (vCPU) is a processor resource assigned to a VM by the hypervisor. Assigning more vCPUs does not create new physical processors; the hypervisor schedules the VM's virtual processors on the host's available CPU resources.

Virtual memory is a portion of host RAM assigned to the VM. The guest treats it as its own memory, but that allocation comes from the physical computer. Giving a VM too much memory can leave the host operating system short of RAM and make the entire computer slow.

Virtual Disk and Optical Drive

A virtual disk is a virtualized storage device used by the guest. It is often stored as a file on the host, although the guest sees it as a disk device. The guest can partition it, format it, and install an operating system on it without needing a separate physical drive.

A virtual optical drive can use a physical CD or DVD drive, or it can be connected to an ISO image. An ISO image is a disk image file commonly used as virtual operating-system installation media.

Virtual Network Adapter

A virtual network adapter is a network device presented to the guest OS. The hypervisor connects it to a configured virtual networking mode, which may provide access through the host's physical network connection. The guest therefore sees a network card, while the actual connection is managed through the host and hypervisor.

Host System and Host Operating System

The host system is the physical computer whose CPU, RAM, storage, and networking resources support the VM. The host operating system is the main OS installed on that computer.

For example, a physical computer might boot Microsoft Windows. Windows is then the host operating system, and the computer itself is the host system. Windows continues to run normally while VMware Player and one or more VMs run as software on it.

The host remains responsible for its own applications and hardware. A VM receives only the virtual resources and device access that the hypervisor makes available. Running a VM does not turn the host's physical components into dedicated hardware owned exclusively by the guest.

Guest Operating System

The guest operating system is the OS installed inside a VM. In the example setup, a Linux distribution is the guest OS.

A guest OS has its own files, users, settings, updates, drivers, and applications. You can log in to it, install software, configure networking, and administer it much as you would a physical computer. Its disk and devices are virtual, but the operating-system experience is still a complete OS environment.

Guest installation media can come from:

• A physical CD or DVD placed in the host computer.
• A virtual optical drive connected to that physical media.
• An ISO image attached to the VM's virtual optical drive.

What Is a Hypervisor?

A hypervisor is the software or firmware layer that creates, runs, and manages virtual machines. It provides each VM with configured virtual hardware and coordinates access to the host's physical resources.

The hypervisor schedules virtual CPUs on physical CPU resources, assigns host memory to virtual memory, reads and writes virtual-disk data, and manages virtual network connections. These resources can be allocated dynamically within the limits configured for each VM and the capacity available on the host.

A hypervisor also helps isolate VMs. Each VM normally operates with its own virtual disk, memory space, and device set. This reduces direct dependence on a particular physical computer and helps prevent ordinary guest activity from interfering with another VM. Isolation is not a complete security guarantee, however. Keep the host and guest patched, use safe configurations, and do not treat a VM as a substitute for security controls.

Type 1 and Type 2 Hypervisors

Hypervisors are commonly grouped into two types according to where they run.

VMware Player as a Type 2 Hypervisor

In this lesson's desktop scenario, VMware Player is a Type 2 hypervisor. It is installed as an application inside the host operating system. VMware Player must therefore be installed on a supported Windows or Linux host before the guest VM can run.

The startup sequence is consequently different from booting a separate physical computer. First, the physical computer starts its host OS. Then the host OS starts VMware Player. VMware Player starts the VM and supplies its virtual hardware, and the guest OS boots from that virtual machine.

How the Layers Work Together

The following stack shows the relationship between the physical computer, the host OS, the Type 2 hypervisor, and the guest:

Guest applications
        |
Guest operating system (for example, Linux)
        |
Virtual CPU, memory, disk, network adapter, and optical drive
        |
Type 2 hypervisor (VMware Player)
        |
Host operating system (for example, Windows)
        |
Physical CPU, RAM, storage, and network hardware

Each layer has a distinct role:

1. The physical hardware supplies the actual processing, memory, storage, and network capabilities.
2. The host OS runs the physical computer and provides the environment in which a Type 2 hypervisor operates.
3. The Type 2 hypervisor creates and manages VMs and mediates their access to physical resources.
4. Virtual hardware gives the guest a consistent set of devices to use.
5. The guest OS boots on that virtual hardware and manages its own applications and files.
6. Guest applications run inside the guest OS, separately from applications running directly on the host.

Multiple VMs can share one host. For example, a Windows host might run a Linux VM and another test VM at the same time. They compete for the host's available CPU, memory, storage performance, and network capacity, so the number of practical VMs depends on the physical computer and their configurations.

Host, Hypervisor, and Guest Roles

Benefits and Practical Characteristics

Portability

A VM is commonly represented by configuration and virtual-disk files. These files can often be copied or moved to another compatible host, subject to hardware compatibility, hypervisor compatibility, storage capacity, and software licensing. Portability is convenient, but copying a VM is not automatically the same as creating a reliable backup.

Manageability

Many VM settings are controlled in software. You can change the assigned memory, vCPU count, virtual disk configuration, network adapter, and installation media without opening the physical computer. You can also start, stop, suspend, rebuild, or remove a VM through the virtualization software.

Isolation

A VM provides a separate environment for its guest OS, files, and applications. This is useful when you want to test a configuration without changing the primary host environment. Isolation has limits: vulnerabilities, unsafe shared folders, exposed network services, or incorrect device settings can still create risks. Patch both host and guest systems and configure sharing carefully.

Testing and Learning

VMs are useful for trying Linux without replacing Windows, learning system administration, testing software installation, and experimenting with an OS configuration. When an experiment is no longer needed, the VM can often be deleted or rebuilt without reinstalling the host OS.

Beginner Setup Scenario: Windows Host and Linux Guest

The conceptual setup sequence is:

1. Start the physical computer and boot Windows. Windows is the host operating system.
2. Install VMware Player on Windows. VMware Player is the Type 2 hypervisor.
3. Create a new VM and choose a name, storage location, and guest operating-system type.
4. Assign virtual hardware, including an appropriate number of vCPUs, guest memory, a virtual disk, and a virtual network adapter. Do not allocate so much CPU or RAM that Windows cannot work normally.
5. Attach a Linux ISO image to the VM's virtual optical drive.
6. Start the VM. The VM boots from the attached installation media.
7. Install Linux onto the VM's virtual disk. Linux is now the guest operating system.
8. Restart the VM and run Linux applications inside the guest environment.

The host-versus-guest distinction is based on role, not on a particular operating-system brand. If the physical computer runs Linux as its primary OS, Linux is the host OS. A separate supported OS running inside a VM is the guest OS.

Troubleshooting Common Beginner Problems

The VM cannot start

Check that the Type 2 hypervisor is installed correctly and that the host operating system is supported. A Type 2 hypervisor depends on a functioning host OS; a VM cannot run as a standalone program before that host environment is available.

The guest installer does not start

Verify that the VM has bootable installation media attached. Check that the intended ISO image is selected and accessible, or confirm that the physical optical disc is present and connected through the virtual optical drive.

The host becomes slow

The guest consumes host CPU, memory, storage, and network resources. Reduce the VM's resource allocations if appropriate, close unnecessary host applications, and avoid running more VMs than the physical computer can support.

The guest has no network connectivity

Check that the VM's virtual network adapter is enabled and that its virtual networking mode is configured appropriately. Also distinguish a guest networking problem from a failure of the host's own network connection.

The host and guest are being confused

Ask where each component is installed. VMware Player is installed on the host OS. The guest OS is installed inside the VM, usually on the VM's virtual disk. In the example, Windows is the host and Linux is the guest.

The guest cannot use a physical device as expected

A guest normally interacts with virtual devices presented by the hypervisor rather than directly controlling every physical device in the host. Check whether the device has a supported virtual equivalent or whether the hypervisor has an explicit device-sharing feature.

Exam-Relevant Notes

• A VM is a complete software-defined computer environment capable of running its own OS and applications.
• The host system is the physical computer; the host OS is the main OS installed on it.
• The guest OS runs inside the VM and uses virtual hardware.
• A hypervisor creates VMs and coordinates their access to physical resources.
• Type 1 hypervisors run directly on hardware; Type 2 hypervisors run on top of a host OS.
• VMware Player is treated as a Type 2 hypervisor in this desktop example.
• Virtual resources are backed by physical host resources and are not unlimited.
• VM isolation improves separation but does not replace patching, careful configuration, or other security practices.

Summary

A virtual machine lets one physical computer provide one or more software-defined computer environments. The host system supplies the physical resources, the host OS runs the ordinary computer environment, and the hypervisor creates and manages virtual hardware. The guest OS boots on that virtual hardware and runs its own applications.

In a Windows-and-Linux example, Windows is the host OS, VMware Player is the Type 2 hypervisor, and Linux is the guest OS. Understanding these layers makes VM creation, resource allocation, installation media, networking, and troubleshooting much easier.

Continue with the virtual machine overview when you need to review the host, guest, and hypervisor relationship.