Question: What are Xen, Vmware, and Hyper-V?
Answer: All three are popular virtualization software. Xen is a free and open source virtualization solution and is therefore included in some Linux distributions. Vmware is a commercial company with many virtualization products. Hyper-V, formerly known as Windows Server Virtualization, is a commercial Microsoft product.
Both Xen and Vmware are cross platform on the host side and client side, and Hyper-V is cross platform on the client side, released with Windows Server 2008. Virtualization allows the user to run guest operating systems within the default system. Theoretically, numerous guest OSes can run at once, limited only by RAM and CPU power.
For servers, virtualization has been an important evener in the web hosting market. Because you can run multiple OSes on one server, you can cut the costs of having to have separate servers for shared hosting users who prefer one over another.