Xen is one of the leading Virtualization software. You can use Xen virtualization to implement HA clusters. However, there are few issues you must be aware of while handling failures in a high-availability environment. This article explains configuration options using Xen:
The idea of using virtual machines to build high available clusters is not new. Some software companies claim that virtualization is the answer to your HA problems, off course that's not true. Yes, you can reduce downtime by migrating virtual machines to another physical machine for maintenance purposes or when you think hardware is about to fail, but if an application crashes you still need to make sure another application instance takes over the service. And by the time your hardware fails, it's usually already too late to initiate the migration.
So, for each and every application you still need to look at whether you want to have it constantly available, if you can afford the application to be down for some time, or if your users won't mind having to relogin when one server fails.
=> Using Xen for High Availability Clusters [onlamp.com]