From Kurt Daniel at Parallels: "Rather than view virtualization as an end in itself, we see it as an enabler for optimizing computing in general to reduce cost and the IT management headache. Virtualization is one element of this. ... Each use case has its own unique set of requirements. Some workloads, such as the I/O intensive applications that run on HP Integrity servers, are more suited to an operating system-level approach like Parallels Virtuozzo Containers. ... Other workloads, such as consolidation projects involving multiple applications running on multiple platforms on the same physical server, are dependent on a hardware-level approach. That is why we are delivering containers and our hypervisor-based Parallels Server combined with management tools and automation."
I like containers for specific environments. Heck, I'm running Virtuozzo with a handful of Debian instances each on two older, non-VT boxes in my production shop; the product might not be as broadly applicable as ESX or Xen, but it gets I/O jobs done fast. If you're looking to P2V a large transactional system or a bunch of similar Linux boxes, take a look.
For the Mac fringe folks out there: No solid date yet on a bare-metal hypervisor for your Intel Xserves. I've got an eight-core in the virtualization test lab running Parallels Server on top of a Leopard build. It works. While Linux, Windows, and Leopard Server VMs run adequately on top of a fat OS, the box is ready to be wiped and rebuilt from the ground up with a "true" hypervisor. My contacts at Parallels are making positive noises ... stay tuned.Parallels other VM product, Virtuozzo Containers, has been vetted on HP Integrity servers all the way up to the monster 64-processor Superdome, leveraging 128 Itanium cores, 2 TB of memory and 192 slots of I/O goodness. Parallels' viewpoint? Virtualization solutions can't be a "one-size-fits-all" approach. That's why the company offers Virtuozzo and the hypervisor-based Parallels Server.