Xen is a tool that allows a computer to run multiple instances of an operating system.
This means Windows and Linux can not only run on the same PC (they can since almost the beginning). This means that they can run at the same time. This technology is truly amazing!
This allows you, for example, to write an article with Microsoft Word in Windows, to release the cursor, to switch the window just as if MS Windows was a simple application of your operating system and to go on your work with Ubuntu or any Linux flavour (that supports Xen, however).
Basically, Xen is based on a guest OS (the dom0). It is in general a GNU/Linux distribution, but some other operating systems (such as NetBSD or OpenSolaris ) supports it. This dom0 is modified, so that it can welcome the guests (the domU’s).
If your hardware is good enough, the domU’s will not have to be changed. You can boot your MS Windows as you ever did (this is the so called full-virtualization – the guests are absolutely not aware that they are running virtually). If not, you’ll have to find a suitable modified distribution (most Linuxes for example) that will allow you to run multiple instances. This is called the para-virtualization.
Even though the performance is a little reduced, it remains excellent (especially for the para-virtualization, because the guests are designed for this technology). Moreover, you can determine what hardware you will assign to which virtual machine VM. Your Windows need a new network card? Just add one! You need to add a hard drive to one of your VM’s? No problem!
This sounds great. Ok, the reality is not as simple as I described above (especially if you have a PC with multiples network interfaces), you will have to find some tricky solutions, work with the command line and edit files, but it the end, it works ! As soon as the Xen developers will build an intuitive graphical interface, this technology will become one of the killer-features of GNU/Linux for the desktop.
The desktop market is the ultimate goal for Linux. But behind the lights, where Linux is not an exception anymore (and little people are aware of that fact), Xen is going to be one of the reasons why system administrators should leave, for example, Netware.
The most striking argument in favour of Xen on the server market is the cost: why should companies continue to pay for 6 average servers when they can build 6 virtual machines on top hardware. This can cut costs in energy, administration, and networking.
Last but not least, Xen is Free software. It means that the basic version is free (whereas Xen Enterprise and Xen Server are not) and you can get the source code, modify it, and adapt it to your needs.
To me, Xen is the ultimate example of how innovative and attractive free software can be. VMware, the proprietary solution is also a good product, but as Xen respects the GNU General Public License, it will be included in a lot of Linux distributions (it is already part of openSUSE) and thus, its community will grow very quickly. It will so benefit from the feedback of millions of the users and developers.
Free software (I will try to give my point of view on it later) does not necessarily mean software communism: XenSource (the company behind it) sells its enterprises versions. I hope they will make money with it and will continue to improve their product -with the support of the open-source community- so that end-users and companies can improve their productivity or have simply have fun with it!