You partially have control over that when you install php with CWP, but you got some problems with your question.
xcache and apc are old (php 5.4 and older) and got replaced by the builtin Opcache.
- You should not have xcache and opcache installed on the same php installation. It will cause conflict.
- Varnish is a separate caching service and it can be "enabled" in the "GUI" only in CWP admin panel. Sadly not in UserCP as far as I am aware.