Xen Project source code can be found on xenbits — the Xen source and distribution server. However, you probably want to go to the download site to find links to the various packages available.
As of February 2013, the Xen Project source code is maintained using the git distributed version control system. Previously codebase was maintained using Mercurial and mirrors of the git trees are still maintained in Mercurial. More information on the available repositories is available on the Xen Repositories wiki page.