The closest airport to Pavia is Milan-Linate, followed by that of Milan-Malpensa.
Currently there is only a direct bus service from Milan-Linate to Pavia. Timetable information of such connection can be found on the Migliavacca website (in Italian) or a copy of the timetable can be downloaded by clicking here.
The best way to arrive from Milan-Malpensa is to take a coach to the Milano Centrale train station and then take a train from Milano Centrale to Pavia: there is at least one train an hour throughout the day. Train timetables may be found here.