What is CentOS Linux?
CentOS Linux is a community-supported distribution derived from sources freely provided to the public by Red Hat for Red Hat Enterprise Linux (RHEL). As such, CentOS Linux aims to be functionally compatible with RHEL. The CentOS Project mainly changes packages to remove upstream vendor branding and artwork. CentOS Linux is no-cost and free to redistribute. Each CentOS version is maintained for up to 10 years (by means of security updates -- the duration of the support interval by Red Hat has varied over time with respect to Sources released). A new CentOS version is released approximately every 2 years and each CentOS version is periodically updated (roughly every 6 months) to support newer hardware. This results in a secure, low-maintenance, reliable, predictable and reproducible Linux environment.
Virtualization is the ability to run multiple operating systems simultaneously on a single computer system. Virtualization has come to prominence in recent years because it provides a way to fully utilize the CPU and resource capacity of a server system whilst providing stability (in that if one virtualized guest system crashes, the host and any other guest systems continue to run).
Virtualization is also useful in terms of trying out different operating systems without having to configure dual boot environments. For example, you can try out Ubuntu Linux without having to re-partition the disk, shut down CentOS and then boot from Ubuntu Linux. You simply start up a virtualized version of Ubuntu as a guest operating system. Similarly, virtualization allows you to run Windows operating systems from within a CentOS system, providing concurrent access to both operating systems. There are a number of ways to implement virtualization on CentOS. Options include, but are not limited to, VMware, Oracle VirtualBox and KVM. One of the most popular technologies on CentOS is called Xen. Red Hat engineers have invested considerable effort into making Xen easy to install, configure and manage on both Red Hat Enterprise Linux and, consequently, also CentOS.
Before plunging into installing and running Xen it is worth taking a little time to talk about the two different types of virtualization supported by Xen and the corresponding hardware requirements.