You have a XenServer 6 or 7, and you have troubles installing CentOS (6 or 7) : the installer loads, but all you get is a black screen …

This is a known issue, and I’m wondering why it wasn’t fixed because it’s there for so long.
When having the boot menu, hit « tab » to show the option, and add the following :

modprobe.blacklist=bochs_drm rhgb

And then it worked !

https://bugs.xenserver.org/browse/XSO-353

https://discussions.citrix.com/topic/364226-centos-7-1503-installer-gives-blank-screen/