What do you think are the main differences?
From what I can tell a President can be elected from a very small pool of carefully chosen and rich individuals. A Prime Minister is chosen by the Monarchs however he can be replaced if he doesn't do his job right. A Dictator can only be overthrown through a revolution or assassination and a King is just a glorified Dictator that supposedly has no power but still reigns without needing to govern, he gets other people do that for him instead.
Is that pretty much it?