Amazon, Rackspace, Eucalyptus, etc. use Xen. Xen is NOT XenServer wrapped as open source. Xen is the base package that is then repackaged by Citrix, Oracle, Sun, Virtual Iron (RIP) and others and sold as commercial virtualization. Xen is not in any way "stripped down", it is actually more full featured as it gets the features first before the commercial variants get them downstream.
Citrix does "own" the Xen project but it is fully OSS and XenServer from Citrix is a derivative of it.
HA is a cloud doesn't necessarily mean anything, though, as the cloud itself is HA and the Xen image can migrate between underlying nodes. So you think about clouds differently than normal virtualization since with normal, in house virtualization, you put together several virt head ends into an HA cluster and that... for all purposes, is an in house cloud.
We run Xen via CentOS (not ON CentOS) here as well. It's awesome.