"very well" is more positive than "fairly well". If someone plays golf very well, they are very good at golf. If someone plays golf fairly well, they are good, but not very good.

fairly well is just a little better than 马马虎虎；we also say 'pretty well', 'pretty good', 'pretty bad' with 'pretty' also expressing a lesser degree - quite, pretty, fairly, rather http://www.englishforums.com/English/WhatDifferenceBetweenQuitePretty-RatherFairly/cvnmv/post.htm