trancejeremy
Adventurer
You are correct that knight and cavalier are the same archetype/theme.
So are the classic cleric and the paladin: in his PHB Gygax describes the cleric as inspired by the fighting orders of the crusades, who were - in self-conception - the pinnacle of knightly chivalry, which is exactly the same archetype as a paladin. Both classes are heavy armour-wearing, heavy weapon-wielding front-line combatants who can perform miracles, particularly healing and turning away evil spirits.
A week late, but I don't think that's true at all. Knightly orders like the Templars were quite different than Paladins, which were meant to represent the Knight Errant found in the tales of Charlemagne (where Paladin comes from) or King Arthur. Clerics were meant to represent the more hierarchical sort of religious knight, and ones that had nothing to do with the secular world (no chivalry, no romantic love, courtship, derring do).
However, I think Cavaliers were introduced because Paladins were the most holy, most good, most chivalrous, not just any old knight errant, they were Roland, Percival, Galahad, Ogier the Dane (of 3 Hearts and 3 Lions), so Cavaliers were added to create the more normal sort, with Paladin being a sub-class. Whereas the cavaliers were more, well, cavalier. Not as strict when it goes to doing good deeds or as chivalrous.