KVM has been very stable and its been great. I am working on the next version of the KVM set up, taking all the good and improving it. I hope new set ups have this by Q3 this year. The basic changes are:
Ability to upgrade CPU and RAM with out a reboot,
Ability to load an ISO and boot from it (say a new install)
Setting time zone in the kvm config
Currently debating the backend still. It may be LVM still, but ZFS is undergoing a serious test. I was hoping to be able to shrink volumes "easily" like openvz/ploop but it may come down to still needing to copy to shrink.