Official Xen Source Releases
The Xen hypervisor is available as source distribution from these download pages. You can find instructions on how to build the Xen source release at this page.
Sources for Xen Binaries
You can get recent Xen binaries as packages from most Linux and Unix distributions, both open source and commercial. Some Linux distributions deliver Xen as a LiveCD, that you can use to easily test Xen.
The Xen Hypervisor is also the foundation of the Xen Cloud Platform, which adds further functionality to the hypervisor and many integrations for third party cloud, storage and networking solutions and comes in an installable image.