I wanted to chime in on the Xen vs. KVM discussions, and give you some food for thought.
I've been using Xen now for years, having replaced UserModeLinux on my personal server, and it's seen a lot of production use at my current job.
That said, KVM is ultimately the right way to go. Constant maintenance of Xen's hypervisor kernel is what causes Linux distributions grief with Xen. Those that keep doing it, do so because it's performant, tested and well-understood.
So you're stuck wondering which virtualization technology you should pick. Here it is: libvirt (with Xen).
Base your toolset, deployment, configuration, and maintenance on libvirt, and use the Xen hypervisor for now.
libvirt provides an easy upgrade path from Xen to KVM once KVM's performance and stability have been battle hardened. Same virtual machine configuration, same provisioning, same disk images. Just grab a new kernel, change which hypervisor you're using and you'll be in the land of tomorrow.
