The panel configure server page says the following about Xcache:
"This only provides any benefit for websites using mod_php or FCGID"
I presume that means that it doesn't work under nginx.
What are my options?
XCache works great with nginx, actually — the description there just never got updated when we added nginx. This forum runs on nginx/xcache, in fact!
(I've just committed a code change to fix the description.)