There is no different between PMOS and NMOS current mirrors in the way they operate. PMOS sources current from supply, while the NMOS sinks current to the ground.
In order to understand more, I suggest you to learn the basic MOSFET functionalities. What I would blindly ask you to do is to watch a few lectures from IIT Madras by Professor Shanthi Pavan :
http://www.ee.iitm.ac.in/videolectures/doku.php?id=ec201_2010:start