You need to see if the xcache extension is installed on the server. You can do that by creating a php script that contains the following, then accessing it in a web browser:
Save that as phpinfo.php and upload it to your server, then use a web browser to go to http://your-website.com/phpinfo.php and search that page for "xcache". If anything comes up, xcache is installed and you can use it.
If you don't find that you have xcache installed, check for "eAccelerator" - if you find it, you can use this instead: eAccelerator for WordPress
If you don't find that, check for "redis" and if you find it, you can use this one instead: Redis Object Cache