Tae kwon do is a Korean style of karate like mu duk kwan, hapkido and tang soo do that Chuck Norris does. Whether the style emphasizes kicks or hand strikes depend on the terrain. It is not good to do kicks in an area that is muddy or icy.
It looks like krav maga was made by combining karate with aikido. A lot has to do with the individual teacher, that you learn from. I learned from teachers that did multiple styles like one does tai chi, kenpo karate and gracie-jiu-jitsu.
I know a medical doctor that says that karate is better for his depression than anything. As far as exercise, you do not need any equipment and you do not need to do it outside like running.
I did karate, yoga and gymnastics and my abs were stronger than anyone else. This guy may be the best that there is. Notice the gymnastics (note that Thai kickboxers use a lot of knee and elbow strikes):
YouTube - Tony Jaa live YouTube - Best Of Tony Jaa YouTube - Tony Jaa
And some Jet Li:
YouTube - JET LI