Create the following registry entry:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Net Framework Setup\NDP\v4\Client\ Install (where Install is a DWORD and should be set to 1)

You are likely to need to add v4\Client and Install

Install the XenTools. When the tools installer tells you to reboot, cancel the prompt and do not reboot. Navigate to C:\Program Files\Citrix\XenTools and delete (or to be safe rename) the following:

XenGuestAgent.exe.cfg
XenDpriv.exe.cfg

(Note the .cfg - you don't want to delete the .exe files)

Remove the .NET registry entry which you added in the first step so Windows does not continue to think it has .NET 4 installed.

Reboot.

You should find everything works fine running on the .Net 3.5 framework. Though this is fairly untested and unsupported by Citrix, so your mileage may vary.