4 gigs of Source ISOs...
We are aware of the requirements of the GPL, and fully support both our commercial editions of Xen and the open-source equivalents. Code flows in both directions.
If you look at the download page for XenServer (http://citrix.com/dl_xenserver_product), you will find 6 source ISOs with about 4 gigabytes of all the open-source code used to create the XenServer ISOs and software development kits.
...including Xen itself :)
Dont forget that the free XenServer Express edition ISO can be downloaded from there and used with no charge or time restriction. If you need more than 4 VMs or multi-server support, upgrade is just a license activation away.