There is also Introductory as from Next year so:
Introductory
Preliminary
Novice
Elementary
Medium
Advanced Medium
Advanced
Prix St Georges
Intermediaire I
Intermediaire II
Grand Prix
Then Grand Prix Special and Kur (although I don't think these are set levels - once you get to Grand Prix you have to qualify for the Special and Kur, I think - Kur you do definately)
Grand Prix special is the highest. It is usually used for the individual tests in finals. The transitions are harder there are more of them and the way inwhich the movements are asked for on the ground plan is more technically difficult.
The Kur at any level has only the main movemets at the level as required elements so could actually be described as an easier Granprix.