Great Question. With Virtuozzo, you have to run the exact same OS in all of the virtual machines on that system. This means that if some applications needed different service packs (or hotfixes), then I would have to put them on different boxes. Also, if you look at the type of Virtualization, Hyper-V is a system level virtualization product, where each Operating System is its own independent OS and isolated entity whereas VZ is OS virtualization where everyone shares the same OS, and so their methodologies are different and VZ seems to be more limited. With Windows Server 2008 R2 with Hyper-V we introduce Live Migration which allows me to move a running VM from one physical system to another with no downtime to the application.