XenServer provides several templates for different operation systems of its hosted virtual machines.

Unfortunately there is no template for ClearOS and so I tried some of the in my opinion best fitting templates.
Here is the list of my attempts:
CentOS 6 (64 bit)
Installation is possible in simple text only mode (black & white). Disabling Viridian doesn’t changed anything. (at the end of the article you will see what I mean with Viridian)
There is a boot parameter “graphical" per default set, but this means obviously something else or doesn’t work. Also I found a blog article where somebody mentioned to add the boot parameter “text”. I was surprised to read this hint – why a parameter named “text” should switch to a graphical environment? And as expected that parameter brought no help.
Debian Wheezy 7.0 (64-bit)
This virtual machine was not able to start at all and immediately returned:
Jan 9, 2015 6:45:09 PM Error: Starting VM 'Debian Wheezy 7.0 (64-bit) (1)' - The bootloader for this VM returned an error -- did the VM installation succeed? INVALID_SOURCE
Unable to access a required file in the specified repository: file:///tmp/cdrom-repo-8Q3Fb0/install/vmlinuz.

Red Hat Enterprise Linux 6 (64-bit)
Same experiences as with CentOS. OK – this was to expect, both systems rely on the same sources.
Windows
Because I could install Windows 10 pretty easy on the Windows 8 template, I thought to give that combination a try. But during the “Storage” step I realized, that the disc must be 24GB in minimum. That was to much and I stopped that try here.
SUSE Linux Enterprise Server 11 SP2 (64-bit)
Same result as with “Debian Wheezy 7.0 (64-bit)”.
Oracle Enterprise Linux 6 (64-bit)
Same experiences as with CentOS.
Ubuntu Precise Pangolin 12.04 (64-bit)
Same result as with “Debian Wheezy 7.0 (64-bit)”. This is not surprising. Debian is the base for Ubuntu.
Other install media
Installation is possible in simple text only mode (black & white).
Other install media – disabled Viridian
As described earlier there is the possibility to deactivate Viridian. I don’t know in detail what happens inside the system, but it makes the difference.
You have to avoid an automatic boot after finishing the configuration. Un-tick this checkbox in the last step and create the new machine:

Now switch to the console of the XenServer

and run this command
xe vm-list
Search for the the UUID of the new machine (here 3a1744ce-cbec-73fa-fb19-ea9d9234c06e).
This UUID you use in the next command
xe vm-param-set uuid="3a1744ce-cbec-73fa-fb19-ea9d9234c06e" platform:viridian=false
Now you can start this virtual machine and you will get an installer with UI

???
This is a note to myself or for you in case you found that article via Google.
I can remember that one virtual machine template or configuration let the installation hang after a few seconds. The last two lines were:
trying to remount root filesystem read write... done
mounting /tmp as tmpfs... done
Use one of the other templates …