Let’s take a closer look at the main players in the virtualization game, along with their respective strengths & weaknesses:If you’re familiar with the concept of how a large hard disk drive is partitioned into smaller logical drives, you have an idea how virtualization works.

VMWare (by VMWare Inc.)

Generally considered the most mature virtualization product on the market today. Also the most expensive. VMware is the closest you can get to a true dedicated server in a VPS environment. VMWare is also a hypervisor.

A hypervisor, also called virtual machine manager (VMM), is one of many hardware virtualization techniques allowing multiple operating systems, termed guests, to run concurrently on a host computer.

Its primary strength is that it’s hard to over-sell a hypervisor because hypervisor-based accounts come with fixed memory limits. With VMWare, Xen and all other hypervisors you can be fairly confident you’re getting what you paid for.

With VMWare (and Xen) you can change your version/distro of Linux, tho your host may only support one particular distro/version.

Some people would think that VMWare is the only software that can truly be used in conjunction with the term “virtualization”.

Xen

Cool name. Also a hypervisor like VMWare.

Like VMWare, over-selling is not allowed in Xen, but the good thing its open source software.

On the downside, this strict compartmentalization leads to less efficient use of server resources (compared to OpenVZ and Virtuozzo).

Many side-by-side comparisons on Xen versus OpenVZ versus Virtuozzo versus VMWare have been performed and debated in online hosting forums. Obviously server configuration plays a big role in these comparisons, and you have to look at WHO is doing the comparison (bias?), and determine whether each server was configured for optimal performance. A small tweak of configuration settings can kill (or boost) performance results.

KVM (open source – Linux-KVM.org)

Stands for Kernel-based Virtual Machine is another hypervisor that is in the mainline Linux kernel. Your host OS has to be Linux but it supports Linux, Windows, Solaris, and BSD guests. It runs on x86 and x86-64 systems with hardware supporting virtualization extensions (Intel VT or AMD-V). It consists of a loadable kernel module, that provides the core virtualization infrastructure and a processor specific module.

OpenVZ (by Parallels Inc.)

OpenVz is is a linux based virtualization platform based on the Linux Kernel. OpenVZ allows a physical server to run multiple isolated operating system instances known as containers. OpenVz can only run linux based operating systems such as Centos, Fedora, Gentoo, and Debian.

One disadvantage of OpenVZ users are not able to make any kernel modifications. All virtual servers have to get along with the kernel version the host runs on. However because it doesn’t have the overhead of a true hypervisor it is very fast and efficient over Xen, KVM and VMware.

This is the open source version of Virtuozzo. It comes without the high-end features that accompany it’s expensive big brother, Virtuozzo.

If you are looking for a fast, affordable and good VPS solution (memory, disk space, bandwidth, etc.) , OpenVZ is for you.

Virtuozzo (also by Parallels Inc. )

Efficient use of server resources is its primary strength.

But Virtuozzo is not open-source. It’s not free.

On the downside, Virtuozzo makes it easy for greedy webhosts to “over-sell” a server, thereby diminishing any performance advantages it might otherwise offer.

This is why hypervisors (Xen, KVM, VMWare) are becoming increasingly popular.

As a point of contrast, with both Virtuozzp and OpenVZ you can only use the version/distro of the operating system (Linux, Windows, etc.) that is used on your host server, because all sites/accounts on the server use the same operating system.

If you wanted to use a different version/distro (operating system), your webhost would have to support that option, and they would have to move your site/account to a different (physical) server.