Communication between vCenter Server and ESXi
vSphere Web Client can be used to communicate with a vCenter Server instance, but it cannot be used to communicate directly with an ESXi host. vSphere Client can communicate with both a vCenter Server instance and an ESXi host.
Access to the ESXi host is provided by a vCenter Server agent named vpxa. The vpxa process is started on the ESXi host when it is added to the vCenter Server inventory. vpxa communicates with the ESXi host agent named hostd. The hostd process runs on the ESXi host and it is used for managing most of the operations on the host. Most commands come from vCenter Server through hostd (e.g. virtual machine creation, migration, removal, etc…).
vpxa acts as an intermediary between the vpxd process on vCenter Server and the hostd process to relay the tasks to be performed on the host.
When you are logged in vCenter Server through vSphere Web Client, vCenter Server passes commands to the ESXi host through the vpxa process. If you are using the vSphere Client to communicate directly with the ESXi host, the communication goes directly to the hostd process.
Here is a picture that explains the communication between an ESXi system and vCenter Server: