I have a feeling you meant to ask about the difference between PAs and NPs (Nurse Practitioners). Their scopes of practice are often very congruous. Comparing PAs and RNs (Registered Nurses) would be similar to comparing, say, a college professor and a graduate TA (I apologize, I know that's not the greatest analogy).
That said, I believe NPs generally obtain an RN degree first.
Edit: Also, don't necessarily mistake PA programs for being "easier." In general, sure, the quantitative academic standards (GPA, GRE [and sometimes MCAT]) are a bit more lax than MD programs; however, there is often more that must be accomplished before entering a PA program. For starters, most programs require additional coursework on top of most premed requirements. Classes like biochem, micro, genetics, statistics, etc are usually needed in addition to the usual bio and chem requirements. It will differ greatly from program to program but most PA schools require a minimum of two semesters of chem, and often more. Some require one gchem and one ochem, others require 2 gchem and at least one ochem, and some require 2 of both like MD programs do. Physics is not always required for PA schools. Again, I'm throwing out generalizations here.
Additionally, PA schools often select individuals that have contributed a GREAT deal of time in the healthcare field already. As one school puts it, "most of our applicants measure their clinical experiences/work not in hours, but in years."