I have used xen over kvm and others for a while. It was what I just started using first years ago. With Xen I could manage domains and my system seemed like a real server. KVM just seemed like a command line trick to me. (I know it is not especially since its integration into the kernel.) xenserver is pretty sweet and there are some big products built around it. Citrix loves it too. OpenStack is neat: http://www.xenproject.org/presentations-and-videos/video/xpus13-rackspace.html I was disappointed when redhat dropped xen :/ I mean now there is an entire project around keeping xen on centos, etc. This one..... In the end xen just seemed more strait forward.