Don't even need an addon, I wrote this bookmarklet:
javascript:part=document.body.innerHTML.substr(document.body.innerHTML.indexOf('flv_url')+8);%20window.location=unescape(part.substr(0,%20part.indexOf('&')))
Create a bookmark with this as the location, then on the XVideos page open the bookmarklet and it should start to download the video (works in Firefox, where you can also add a keyword to the bookmarklet, then type that in the address bar).