An extremely simple, and hopefully useful plugin. It's designed with compatibility in mind, so there's no fancy variables and things on the permissions.
Players with a certain permission node will get a certain multiplier bonus on any XP they pick up while on your server. This plugin has 0 configuration files, 0 commands, just a grand total of 9 permission nodes to choose from.
As you can see, these are the permission nodes. You don't need a multiplier of 1, since this is what you get anyway, and if a player inherits two of the permissions the higher one will always be set.Code (Text):xpmultiplier.2
Personally I recommend not really going above 3x-4x for normal players, for the sake of balancing the game. Players can get a ton of xp from mob grinders already.
Disclaimer: This plugin was not made by me, but for me. I paid for this, and am letting the public make use of it. Therefore, treat it 'as is' without ANY support given should it not work.