What is the difference between RNAV and RNP?
RNP is a superset of RNAV requirements. It applies and communicates a performance specification for RNAV that may vary by location rather than by equipment.

On board performance monitoring in RNP requires an alert when the system cannot guarantee with sufficient integrity that position meets required accuracy. Required accuracy can vary. A tight approach may require 0.1nm accuracy while 5-10nm over oceans might be acceptable. RNP requires communication of a numeric qualifier for accuracy. RNAV has no such mechanism.

Confusion is understandable as the terms are sometimes used interchangeably. ICAO is arguing with FAA and other countries to rename RNAV approaches as RNP approaches. The ICAO position is more clear, the FAA position supports existing equipment and training. Everyone believes a navigation performance spec needs to be communicated to pilots in charts and equipment, but there is no clear answer as to how to do it at this point.