I'm planning on taking the TGV from Paris to Avignon and then continue to Aix en Provence. Is there train service or do I need to rent a car?
There's direct, local train service. You don't need a car between those two.
Thanks, Bets
Sorry, ignore my previous response, the way you expressed the initial question confused me. There is no need to change at Avignon, the easiest way is to get a TGV from Paris to Aix. The TGV station at Aix is a little way out of town and the bus to the city centre takes 15 minutes.
If you are staying in Avignon and then going on to Aix, there is no direct rail service between the two towns and if travelling by train you need to go all the way to Marseille and back. Regional bus route number 23 operates six journeys a day usually and four during the school summer vacations: timetable at http://www.info-ler.fr/doc/ficheshoraires/20122013/bat2013/fh23.pdf