Hi....
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.
Vijay Bhardwaj