The two remaining solutions we looked at are Xen and kvm.
Xen has of course the advantage of the matured. Also, we have experience with running xen servers - and with the issues that can happen, like the chances to disconnect dom0 from xend, and then reboot the server the hard way. However, the most serious disadvantage is that development has practically stalled with the 2.6.18-kernel. Of course, even of today one could install a new server based on Etch, but that doesn't really feel right. There is some development ongoing to run domUs with newer kernels (like in Lenny), but there isn't currently any new kernel available for dom0.
kvm is a more recent addition to the virtualization camp, and is basically "qemu on steroids". All looks rather promising, development happens with the recent kernels. However, kvm lacks a few features of e.g. Xen.
This includes the ability to reboot dom0 (and the hardware) and just let the domUs survive. Or to have a nice management script where one could just say "xm shutdown $domU", and have basically the power button be pressed on the virtual machine. Or to just attach and detach to the virtual console whenever one wants. Nothing of that is impossible with kvm, one could attach the command-terminal to some pipe, and the linux console to some other, and attach and distach via own scripts. But - all of that should be expected to be available from some solution that calls itself enterprise ready. (And - writing own scripts has always the possibility to make own mistakes.)
However, among all the worst possible issue is that kvm is underdocumented (or rather: There are lots of different places where some parts of the documentation is hidden - including the great remark in the man page "The other options are similar to those of qemu.").
So, what to do? Invest more time into a solution that seems like a dead end. Or put up with the incompletness of another solution?