The Hypervisor is developed by a world-wide community of individuals, researchers and employees of companies and that follow the Xen Project Governance process based on openness, transparency and meritocracy. The project is supported by the Xen Project Advisory Board made up of project member companies that fund the Xen Project. You can find a recent contribution breakdown in the Xen 4.3 and Xen 4.2 Contribution Acknowledgments.
Introduction to the Hypervisor
What's in Xen 4.3
Xen 4.4 Planning Update
Xen Releases are developed on a 9 months release cadence and follows the process outlined here.
Getting the Hypervisor
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 licensed under the GNU General Public License (GPL2).