I like KVM
It works and behaves like another process. One can easily use it in an adhoc manner from their desktop or shell. XEN on the other hand requires significant changes to the system setup & kernel. I spent a lot of time fiddling with XEN on my desktop, but there were significant incompatibilities with my graphics card, it was far from plug and play.
For large scale server deployments with domain experts this may be ok, but there is no question KVM is more intuitive and user friendly.
Unfortunately, all my KVMs were bricked by the most recent kernel update from ubuntu. The virtualized systems would crash a couple seconds into execution. Meanwhile, KVM's predecessor QEMU, which uses emulation instead of virtualization, continued to work.
While this bug was disappointing, I believe red hat's distribution is little more stable (less cutting edge) than ubuntu. Anyone with commercial support should be ok.