Xen Virtualization is a new book from Packt that helps Linux administrators to use Xen virtualization for development, testing, virtual hosting or operating systems training. Written by Prabhakar Chaganti this book is a practical guide for supporting multiple operating systems with the Xen hypervisor.

Xen was originally developed in 2003 at the University of Cambridge Computer Laboratory and users now have the choices of both commercial and free versions of the Xen hypervisor. The commercial versions are built on top of the open-source version with additional enterprise features. In this book Chaganti explores and uses the open-source version of Xen.

Xen is an open-source paravirtualization technology that provides a platform for running multiple operating systems on one physical hardware resource, while providing close to native performance. Xen supports several operating systems like Linux, FreeBSD, Windows, and NetBSD. It enables the user to easily test, deploy and run their software and services on multiple operating systems with resource isolation and great performance. It is also a terrific way to consolidate servers, save hardware and maintenance costs, and minimize downtime. Xen is one of the most popular open source projects in the world and vendors like IBM, Sun, HP, RedHat and Novell are working on integrating Xen into their Linux servers.

The dust has yet to settle from the recent announcements that Linux will switch it's process scheduler, but we're already bombarded with interesting news that the upcoming 2.6.23 kernel release will also feature no less than two new virtualization frameworks.

The smaller one is called lguest, "a simple hypervisor for Linux on Linux. Unlike kvm it doesn't need VT/SVM hardware. Unlike Xen it's simply 'modprobe and go'. Unlike both, it's 5000 lines and self-contained.". The lguest patches are written and maintained by Rusty Russell who explains, "lguest is designed to be a minimal hypervisor for the Linux kernel, for Linux developers and users to experiment with virtualization with the minimum of complexity. Nonetheless, it should have sufficient features to make it useful for specific tasks, and, of course, you are encouraged to fork and enhance it.". (commit)

The following paragraph shamelessly stolen from kvm FAQ, but I suppose this information should be quite useful to anybody who is still deciding which product to use.

Xen is an external hypervisor; it assumes control of the machine and divides resources among guests. On the other hand, kvm is part of Linux and uses the regular Linux scheduler and memory management. This means that kvm is much smaller and simpler to use.

kvm only run on processors that supports x86 hvm (vt/svm instructions set) whereas Xen also allows running modified operating systems on non-hvm x86 processors using a technique called paravirtualization. kvm does not support paravirtualization for cpu but may support paravirtualization for device drivers to improve I/O performance.

Virtualization is a hot topic these days. With hardware getting more and more capable, by means of cheap multi-core processors and gobs of memory, we can expect virtualization to become only more important in the coming years. Virtualization promises reduced costs for IT organizations, both hard (machines, power, cooling) and soft (admin and operations personnel).

There are lots of products on the market already, but two that draw the most attention are open source Xen & VMware family of products. VMware is the pioneer of the virtualization on the industry-standard hardware (having been founded in 1998) and is currently offering many inovative products, some of which are free to download and use. Xen, on the other side, is under heavy development, but is already promising unprecedented levels of performance.