Vanguard is probably not suited for me as I am a terrible kiter. So it's either Shadow or Guardian.
Vanguards aren't kiters. They actually operate best just standing toe-to-toe rather than arbitrarily trying to kite their targets. I'm curious how exactly you arrived at the conclusion that VGs must kite to be effective tanks. If you really just don't want to play a VG for reasons other than "I think they're supposed to kite, and I don't like doing that", then ignore any mention of them to follow. If the only reason you didn't want to play one was because you thought they had to kite, feel free to consider them (they actually make an excellent "my first tank" because they're simple and easy to play which allows you to focus on the mechanics of tanking rather than the mechanics of your class).
What are the main differences between them?
Guardians have the worst threat and damage of all of the tanks. Shadows have the best damage and threat of all of the tanks. VGs are in the middle.
All of the tanks have pretty much the same survivability when played proficiently. When played poorly, a Shadow has the worst survivability, a Guardian is in the middle, and a VG has the best. When played proficiently, they're all pretty much the same. When played *well*, Shadows are the best, Guardians are in the middle, and VGs have the worst (though it's still perfectly serviceable for everything endgame).
Guardians are, pretty much, universally agreed to be the hardest tanks to learn to play proficiently while VGs are the, de facto, simplest tank outright; Shadows are somewhere in the middle. When trying to play the class at the absolute top tier of performance, Shadows are the hardest, Guardians just below, and VGs are still pretty much the same (this stems from Shadows having a lot of active mitigation and attack interaction with their attack priority whereas Guardians and VGs are pretty much static once you've learned how to play them, not to mention the power and specificity of Shadow CDs compared to the general applicability of Guardian and VG CDs).
In PvP, tanks are generally considered not that impressive. Guardians, as pure tanks, are probably the most useful because of their high mobility (Force Leap and Guardian Leap) and utility (slows, stuns, etc.). Shadows are generally preferred to run as the hybrid tank spec (23/1/17, generally) while running in DPS gear which gives them a great combination of utility (instant-Force Lift, 30m 3 target hit, stealth), impressive damage (PA proc for instant crit with Project), and adequate survivability (tank grade K/E DR coupled with Shadow CDs; the loss of shield and defense rating doesn't mean much because the ratio of M/R to F/T in PvP is vastly different than in PvE). VGs as tanks aren't really considered all that useful since they don't get impressive burst or mobility and their only real utility is Harpoon, which can be brought along with better damage and utility, by the hybrid Shadow. Of course, for PvP, you can always swap to an Assault VG which is considered to be one of the best burst DPS classes there is.
As to the ability to switch to DPS, all 3 classes can do it, but VGs are probably going to get the most bang for their buck out of it: Assault VGs are top tier DPS. Shadow and Guardian DPS are respectable enough that you're not going to get kicked out of raid groups, but they're not top tier either.