Difference between xen Server <-> Xen Project Hypervisor <-> XCP-ng

[Uwe Faber <uf@xxxxxx>]

So my i introduce myself,

i worked with xen Server or Xen Hypervisor since xenserver 5 until 8.2
It was licensed by Citrix with support.  So the future in 2018 of 
xenserver seemed not so clear so we changed to nutanix and had only 
legacy VMs on xen server 8.2 wirh no support contract.
Now i am offered a project to migrate  XEn Desktop and Apps from VMware 
to xen, because auf the license costs.
So Xen Server 8.4 is able to work with VDI, but this versions had 
License costs therefore i had following questions:
is the use of xen Hypervisor project recommanded from citrix ? Can i use 
this Hypervisor ?
what is the differnce between XCP-NG and Xen Hypervisor in this context ?

Regards

Uwe