Area navigation (RNAV) - A navigation method that allows aircraft to operate on any desired flight path within the coverage of ground- or space-based navigation aids, or within the limits of the capability of self-contained aids, or a combination of both methods.
RNP is Rnav with addition of an onboard performance monitoring and alerting capability. A defining characteristic of RNP ops is the ability of the a/c navigation system to monitor the navigation performance it achieves and inform the crew if the requirement is not met during operation.
The difference is very simple RNAV being area navigation and RNP being the monitoring system to check whether you are achieving the required accuracy or not.