The D at the end means dual motor, if it lacks the D, it's rear wheel drive. Currently only the 70/75 is offered with rear wheel drive, but all the early Model Ss were rear wheel drive only. They introduced the dual motor version in late 2014.
The P is the performance model and it's somewhat more expensive. It has little difference from the 90D except for a larger rear motor and different firmware installed. The P90DL has the Ludicrous upgrade which gives the maximum acceleration of any Tesla (1.1gs, 0-60 in 2.8s). The P90DL is very popular with people who like to do drag racing with street cars and YouTube is full of them.
There is no P75DL, because of the smaller battery capacity in the 70/75, the cars are software limited to lower performance than the 90 KWh battery cars. Though wk057 (here on the forum) who has taken apart a number of wrecked Teslas and has learned a lot about how they work has said the 70D/75D and the 90D are identical except for the battery and software. Put in a 90 KWh battery and the 90D software and a 70D or 75D becomes a 90D.
The 70 is in the process of transitioning to the 75 right now. They haven't built any Model Xs with 70 batteries and all 70s delivered today have 75 KWh batteries that can be upgraded with a call to Tesla and $3000. I suspect they will drop the 70 KWh battery in a few months. They did that with the 85->90 KWh battery. It was a paid upgrade for about 8 months and they finally dropped the 85 and only offered the 90 earlier this year.
Other than never building a 70 KWh Model X, the batteries and drive components are identical between the S and the X. They had different chargers modules for a while, but the S got the Model X charger in April of this year.