A hypervisor is a piece of software that enables a user to create and runs one or more virtual machines stimultaneously. A hypervisor is also known as the virtual machine monitor (VMM).
One of the key functions that hypervisor provides is isolation, meaning that a guest cannot affect the operation of the host or any other guest, even if it crashes.
Hypervisors can be of two types: Type 1 and Type 2.
Type 1 – also known as native or bare metal hypervisor, this type of hypervisor runs directly on top of the physical hardware. Each virtual operating system runs atop the hypervisor. Examples of bare metal hypervisors are the Oracle VM server, Vmware ESX/ESXi and Microsoft Hyper-V.
Type 2 – also known as a hosted hypervisor. This type of hypervisor is installed as a software application on an existing host operating system (OS). Example of the hosted hypervisor are Oracle VirtualBox, Microsoft Virtual PC, Vmware Server and Workstation.