hostd is the daemon for direct VIC connection (when you use VIC to connect to your ESX).
vpx is the original name of vCenter Center, so:
vpxa is the VC agent (ESX side)
vpxd is the VC daemon (VC side)
In ESX you have only hostd and (if you have vCenter) vpxa.
These are daemon (services) for remote management:
hostd is used to remote management using VIC
vpxa is used by vCenter (the vpxd part of vCenter) to remote manament
VPXD-It is Vcenter Server Service. If this service is stopped then we will not able to connect to Vcenter Server via Vsphere client.
VPXA-It is the agent of Vcenter server. also known as mini vcenter server which is installed on the each esx server which is managed by Vcenter server. What are the management action we are performing on top of the vcenter server. (Like:- Increasing/Decreasing RAM & HDD, Making any type of changes in cluster,
doing vmotion. This agent collects all information from the vcenter server and pass this information to the kernal of the esx server.
HOSTD- This is the agent of ESX server, here VPXA pass the information to the HOSTD and hostd pass the information to ESX server.
Hope this one give you clear explanation about the difference in between each of them.
Nicely explained Vijay
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Hostd is deamon or service runs in every ESXi host and performs major tasks like, VM power on. But when ESXi host joins in a vCenter server Vpxa agent gets activated and talk to Vpxd service which runs in vCenter server. So the conclusion is Vpxd talks to Vpxa and Vpxa to hostd, this is how Vpxd (vCenter) talks to hostd via Vpxa.