Virtuozzo: Xen, Microsoft and VMWare are reinventing the wheel with their hypervisor/para-virtualization technology, doing the things operating system has always been doing.
Essentially, the hypervisor is a lot like an operating system. It shares CPU, memory and I/O between Virtual Machines -- it includes an SMP scheduler, memory manager, I/O subsystems, plus its own driver. In addition, the hypervisor needs to efficiently support PAE, NUMA, thread libraries and other low-level mechanisms.
So the conclusion is?
Frankly, it's not like I'm going to lose sleep over VMware, Microsoft and Xen spending a lot of money and time of their best people on re-implementing what is essentially an OS kernel just below the actual kernel. Needless to say, Virtuozzo does not re-implement anything. It uses the OS and drivers directly and automatically takes advantage of all improvements made to the OS kernel.
That is what you expect when you try to read virtualization-related news from a vendor's website -- full of self-promotion and spreading FUD.
Disclaimer here -- I am not a virtualization expert, but a user who have used and deployed both technologies, OS-level container and hypervisor. I don't deny that Virtuozzo/OpenVZ are having less overhead -- VZ is only a thin layer of the current running OS, whereas in the case of Xen you have a regular OS on top of hypervisor. Even more complicated for VMWare when it needs to emulate the whole hardware.
However in the case of Xen, dom0 sits on top of a patched Linux or NetBSD which already has all the optimisation for the hardware. DomU nodes only need to optimise for the hardware that has been virtualized. There is no "special hypervisor OS" that needs to re-implement its own device drivers.
From user's point of view however, there is a huge difference between a container VPS and a hardware-virtualized VPS. If you have experience with OpenVZ or Virtuozzo, you know how it feels -- you know that you are in a VPS. VM parameters reported by the kernel is inaccurate.
top are useless. VZ's UBC is a mess to work around. Where can I tune my kernel's "swapiness"? There are just so many things you can't do where you can on a dedicated server and on a Xen or VMWare VPS.
It seems OpenVZ/Virtuozzo is only good for web hosting, where you have an array of cheap oversold VPS all running CentOS 4 and Plesk. Wait! What else are SWSoft selling?
Hardware is so cheap these days, and you basically don't care whether it is 1% or 3% overhead on your multi-gigahertz multi-core big iron. A full-virtualization makes those big hardware really useful to run multiple operating systems, and I am sure SWSoft knows it. Too bad Virtuozzo is a container technology and can only run on a few specific patched versions of Linux.