Hi, I.B.
"Kuai le" is easy; it means "happy/happiness". While "xing fu" is very often translated into "happy/happiness", it's actually an understatement. If you ask me, "xing fu" is more complicated or greater than "kuai le".
I personally think a better translation for "Xing fu" could be "fortunate(ness) and blessed(ness)". It can be roughly seen as a mixture of "happy" and "satisfied/secured/complete". Or in short, you can use "xing fu" when you think "I'll ask for no more".
For example, you will feel "xing fu" when you can get in a heated room in a blizzard. For someone with insomnia, he/she will feel very "xing fu" when he/she can finally get a good night's sleep. Many people will feel "xing fu" when they can marry someone they really love.
Hope this helps.