3.5 had a fixed table with protected (non-OGL) hit point tables. Pathfinder was obviously not allowed to copy that, but they simplified the system a bit, while making it harder for lower level characters to close in on higher level characters.
An in-depth explanation can be found in this article by Jonathan Drain at D20 Source:
The big difference with the Pathfinder/UA experience system is that instead of earning a different amount of XP from a creature depending on your party’s current level, you gain the same XP amount regardless of party level, but higher level monsters give much more XP, and you need more XP to level up. The overall effect is that the XP numbers get much higher (millions of XP to reach level 20), but players still level up at the same rate as normal.
The main benefit here is that DMs can combine monsters in encounters more easily and calculate XP rewards more easily.
Another change peculiar to Pathfinder RPG is the choice of three XP rates: slow, medium, and fast. Some groups prefer to level up frequently, while others prefer slower levelling.