Citrix has a number of virtualization solutions. XenDesktop and XenApp are two of them. The main difference between them is that XenDesktop has one individual desktop (and therefore own VM) only for each user and XenApp is a shared desktop model. The technical difference between the two is that XenApp is based on MSFT Terminal Services and hundreds of users can get virtual apps and desktops from a single VM. XenApp is fully supported by Citrix on AWS today.
Citrix XenServer (just like VMWare and MS Virtual Server) will not run on AWS/EC2. This is the case even though XenSever (as the name implies) is based upon the same open source kernel as AWS hypervisor.