Like Microsoft's Virtual PC 2007 and Virtual Server 2005 products, XenServer (recently acquired by Citrix) lets you run multiple virtual machines, simultaneously, on the same host system. Like Microsoft and some VMware products, it can also be downloaded for free and used to host virtual machines running either Windows or Linux. But there most of the similarities end, with a lot more to XenServer than the majority of other virtualisation products.

The biggest difference is that XenServer is what's called a hypervisor, which means you don't need a host operating system on the server to start with. Simply download the code, burn it to a CD and boot from it to load up all the software needed. And, because there's no host operating system in the way, you should end up with better performance plus far fewer security worries.

On the downside, XenServer is a 64-bit only solution, which means you'll need a PC or server with one or more 64-bit processors, either from Intel or AMD. Processors with Intel-VT and AMD-V virtualisation extensions are also required, although only if you want to run Windows guests, as Linux can be supported on older chips without the enhancements.

Installing the software is reasonably easy and much like configuring a Linux distro, which is unsurprising as that's what it's based on. And once installed there's no facility to do very much locally, with a Windows-based console (XenCenter) used to remotely manage XenServer.

We found the XenCenter console easy to use and were quickly able to create new virtual machines using the templates provided. Virtual machines can be configured with either 32- or 64-bit guest software, with support for Windows 2000 SP4, XP and Server 2003 plus most leading implementations of Linux as standard. Unfortunately Vista isn't supported in the current (v4) release but will be in v4.1, which was in beta during our tests and will also deliver additional scalability and functionality enhancements.

Rather than use the templates, you can also create virtual machines from scratch, with the usual options to specify the amount of memory to allocate, disk space and so on. A VNC-based monitor is provided to connect to the VMs, although for Windows we preferred the optional Remote Desktop interface as it's a lot snappier. You also get plenty of virtual networking options plus the usual tools for installation into guest machines to enhance the remote screen, keyboard and mouse interfaces.

Tools to backup virtual machines, similarly, come as standard along with others to copy and clone VMs. There's also a physical-to-virtual conversion tool for use with Linux hosts. A tool to convert Windows servers isn't included, but can be had from PlateSpin (now part of Novell), added to which XenServer uses the same Virtual Hard Disk (VHD) format as Microsoft's virtualisation products.

We tested the free Express Edition of XenServer which is limited to just two physical processors and 4GB of memory, plus it only lets you run up to four virtual machines at any one time. This shouldn't a problem for most home or small business users, but upgrade to the Standard or Enterprise editions and there are no limits in terms of concurrent VMs, plus the host server can have up to 128GB of RAM with up to 32GB per guest. The Enterprise edition can also make use of shared iSCSI or NAS storage plus you get a tool called XenMotion which allows virtual machines to be migrated between servers without powering them down.

There's no need to upgrade the software, just install a new licence, but the commercial versions are far from cheap. A 2-socket Standard Edition licence with bundled support, for example, starts at $600 per year. Dell has also announced the availability of XenServer as an option on its PowerEdge servers.

With no need for a host operating system, the XenServer hypervisor makes best use of PC or server hardware to host multiple virtual machines running either Windows or Linux. Guest operating systems can be either 32-bit or 64-bit, the software is easy to install and available for free download. The free version has some limitations but is fine for home and small business use.