We currently use openvz. Previously we used Xen and then BSD jails. Every one has their ups and downs. As many people will tell you, timing is critical in VoIP applications and by using OpenVZ there is only one kernel loaded and the VMs all run bare metal. This is a much more efficient way of running several VMs if they are all of a similar OS because you don't need to waste a bunch of RAM and CPU cycles just loading 30 copied of the kernel into memory and dealing with paging etc.. Also this makes administration much easier because of separate filesystems for each VM, the root of each VM is located somewhere in the host's filesystem. This is a big plus when you go to back up your hardware node. What we do is put all the VMs on an LVM partition, make a snapshot and use rsync to back up just the changes. It was a major pain with several independent LVM partitions or "virtual" file based partitions in Xen. KVM or any true virt platform would require the same, a dedicated partition for each VM. And CPU scaling works well with openvz allowing us to save on the electric bill and cooling costs. The downside is if there is a kernel panic, its one kernel and the whole world goes with it. BSD Jails was cool but didn't allow the per client resource control we were looking for. Our experience with openvz has been solid with a couple of machines up well over a year. We currently use Centos but we've modified it to the point where its basically our own in house distro with updates to some of the core packages since RH stays so far behind. (for good measure :-)) This allows us to pick and choose which packages we'd like to update and we make available our own repo which a simple yum update pulls down everything from a local mirror. If you are interested:
http://repo.5ninesolutions.com. Having said that, there is a lot of time that goes into maintaining some of the out of distro stuff so we are currently looking at something that stays a little more current.. I.e. RH 6 has been out since Nov and I haven't seen a beta for Centos 6.. but thats a whole can of worms..