"3. Virtual Iron - Based on Xen's virtualization technology, Virtual Iron is an up-and-comer in this space. Though I haven't tried their product(s) directly, I hear very good things about them from other virtualization professionals. I'm not qualified to say whether or not they offer anything above what Xen alone offers--that would take some investigation. Perhaps some of you who've tried Virtual Iron can use the comments section to inform me of your experiences.
"4. OpenVZ - OpenVZ, Virtuozzo, and Parallels all sprang from the same origin: SWSoft, which is now Parallels, Inc.
These products are all based on operating system-level virtualization or containers. Containers are actually a type of BSD Jail or chroot jail. This is a very efficient and secure type of virtualization where each virtual machine is really just an extension of the existing system. It's a bit hard to explain in such a small space but compared to other types of virtualization, the costs are nil, the performance is native, and the number of virtual machines can reach into the hundreds. The only limitation is that you have to run the same operating system and kernel for all virtual machines."
