Originally Posted by TonyMeister
I did boxing all my life, but now I want to try a martial art. Although, I did one year of karate once, but only one year of karate doesn't really count does it?
Krav maga and muay thai are "hot" right now and kickboxing is very much like muay thai (is there any difference???).
But which one of these three, would you chaps would recommend?
I'm not interested into spending a whole decade pursuing a black belt, I just want to learn how to defend myself and stay fit.
Any of them will be ok for fitness.
All will give you some form of skills for self defense; beware as many martial arts these days are taught more for sport more than self defense.
From you limited list above, KM probably best fits your requirements but it also depends upon the school, the teacher, ...
If you want hard core self defense, have a look for any CQC (close quarter combat) schools near you.
Check out http://www.faqs.org/faqs/martial-arts
which will give you some more info on the arts in your list.