Good question. I think the difference is slight to nonexistent (especially in English translation). The two roots may suggest shades of meaning: покой - a state of calm, lack of motion, inaction, quiet (check out the meaning of покойник, покойница); волна - a wave. Волноваться has a couple other meanings in addition to "worry": (1) only in the third person, to wave as in Море волнуется. (2) first and second person only in the plural, to massively protest, express discontent with something.