Normally you can install things on a shared host, as long as it does not require you to have root access to the server.
Sorry I don’t have experience with xcache though.
You can generally install xcache in your home directory and update php.ini to point to is, but not all hosts allow it (and you need shell access), while on the other hand, so hosts provide it as an option (I’m pretty sure DreamHost does, but you have to ask for it). That said, I’ve not done this (only APC, and not on a shared host).
If you’re worried about performance, don’t use a shared host and/or look at WordPress caching plugins and other WordPress performance optimizations.
Also, I’d venture to guess that this request is outside the scope of this forum…
If you want to reduce CPU usage on your shared server, you can use other plugins too.
W3 Total cache :wordpress.org/plugins/w3-total-cache
WP super cache :wordpress.org/plugins/wp-super-cache
Above plugins are excellent and very famous. Many wordpress users are using the, I suggest you to have a look at them too.
I use W3 Total cach already.
I’ve often had luck using DB Cache Reloaded Fix when the normal page caching plugins didn’t work as well as I liked.
