We use Xen as our platform for virtualization.
Xen is a free virtual machine monitor for IA-32, x86-64, IA-64 and PowerPC architectures. It is software that runs on a host operating system and allows one to run several guest operating systems on top of the host on the same computer hardware at the same time. Modified versions of Linux and NetBSD can be used as hosts. Several modified Unix-like systems may be employed as guest systems; since Xen version 3.0 unmodified versions of Windows and other systems can also be used as guests on certain hardware.
Xen originated as a research project at the University of Cambridge, led by Ian Pratt, senior lecturer at Cambridge and founder of XenSource, Inc. This company now supports the development of the open source project and also sells enterprise versions of the software. The first public release of Xen was made available in 2003.
Xen has several advantages over other virtualization methods like VMWare, Virtuozzo and linux-vservers. The main advantage is the performance, Xen servers perform very close to native because is uses a hypervisor layer instead of an emulator to run the client system.
Xen misses one feature that all other virtualization techniques have: memory sharing. The resources a guest domain receives from Xen are dedicated to that virtual machine. While this could be a nice feature to have (it would enable us to put 20 * 512MB hosts on a single 4GB machine,) the benefits for you as a user are great. Most of the virtual servers sold on the market today have (semi-) shared resources and performance is often very low. Even if we wanted, and we don't, we could not overbook a server at the cost of performance for you.
If you have more technology related questions, please also see our Frequently Asked Questions