i’m not sure if I understand you question but if you want to install ESX and Xen on the same machine it wont work. ESX is taking all the place and will not let you do thet.
Also why do you want to do this. try both product in their environement OK but not at the same time on the same machine that is not very productive.
I assume you are only doing this for testing purposes. It is possible to have the 2 desktop versions installed on a single OS, but I would recommend you choose one or the other.
You cannot combine the server versions to run on a single machine.