Interestingly, XCP-ng will be based on XenServer 7.4, which Citrix announced on March 3rd, 2018.
Starting with XenServer 6.4 will therefore mean XCP-ng has all the latest toys to play with.
XCP-ng, the effort to revive an open source version of XenServer, will go ahead after crushing its crowdfunding campaign. The project's Kickstarter sought €6,000 but ended up with €38,531 from crowdfunding contributors. Project founder Olivier Lambert wrote to backers with news that their donations, plus more money from as-yet …
Sorry but, I think you may be confusing XenServer and Xen.
Xen the hypervisor is a Linux foundation project, it is indeed used by Amazon but the API Amazon developed is not open source.
XenServer is a Citrix product and an open source project (impossible to build by anyone else than Citrix team but still open source) - It includes Xen hypervisor and the Xen API (basically, it's a turnkey Xen)
XCP-ng forked it because the XenServer project is only open source by name and only contributor to the project is Citrix. Honestly, the lack of documentation and transparency on that project make it impossible for anyone else to build a proper XenServer from the sources.