During the installation of a new VM.
If the VM is HVM type then after the welcome screen every kind of screen with has graphic will stuck in black.
It seems is loading... but is not.
You can press keys and they'll afect the screen (even if you'll not notice).
To solve, close XenCenter and reopen Xencenter.
Go to "console" and you'll see that is working and everything you have digit is in the screen.
It's an issue that show up everytime I create a new installation from zero.