How Xen Works

Xen is actually a very small operating system that has the sole goal of managing the resources of virtual machines. On top of the Xen OS runs what would previously have been called the host OSthe main operating system for the machine. Unlike VMware, the host OS (known as domain 0 or just dom0 in Xen terminology) is a virtual machine, but has special privileges assigned to it so that it is more responsive.

The domain 0 VM is where you control Xen, and where you start other guest VMsknown as unprivileged domains or domU. You can start as many as you want, with the only really limit being the amount of RAM in your machine. Because the domU OS is fully aware of its status as a Xen virtual machine, Xen lets you change the amount of RAM in a VM while it's running, with the exception that you can't go above the initial allocation of RAM.

Fedora Linux specifies a minimum system requirement of 256MB RAM, which means that if you are to run Fedora on top of Fedora, you need at least 512MB RAM. Keeping in mind that Xen uses a very small amount of RAM for itself, you should ideally have at least 768MB of RAM to be able to run two operating systems side-by-side at full speed.

Note that if you have tried Xen on old versions of Fedora, you no longer need to disable SELinux to get Xen to work.