I have seen Ubuntu, especially 10 and 11 doing amazing blunders in conjunction with kvm and libvirt, both as hosts and guests. Internet forums are full of reports where VMs lock up, die and generally misbehave, that could not be reproduced on Fedora or RHEL. This is something very easy to prove, and not written in order to start yet another distro war (even though I am definitely not a fan of Ubuntu as a server of any kind)
* The VMs themselves are also running Ubuntu 10.04. It's worth noting
that these systems haven't had security patches applied for a long
time, despite my best efforts (the customer is holding this up a bit).
I'm hoping that once these updates happen, the problem will go away,
but in the interim, I'm looking into other possible causes.