Access to the ESXi host is provided by a vCenter 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 to manage most of the operations on the host. Most command some 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 to vCenter through vSphere Web Client, vCenter 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.
