Virtuozzo is based on OpenVZ, it is not an apart virtualization technology. There are only Xen, OpenVZ and HyperVM. Virtuozzo, VDSmanager use this technologies for Linux and Win. VDSmanager use also own FreeBSD virtualization technology.
We migrated straight from hypervm to VDSmanager and love it. It has a customer portal so they can log in and reload there vm's, reboot, view network graphs ect. It has central management of multiple servers. It also seems to be updated frequently and I have never had problems getting support.