By Plane:

Pavia can be easily reached from Milan, which is served by two main international airports: Milano Malpensa (MXP) and Milano Linate (LIN). From both airports, it is possible to reach Pavia by train or bus.
From Linate Airport, you can take a direct bus to Pavia (approximately 1 hour) or reach Milan Central Station by shuttle and continue to Pavia by train.
From Malpensa Airport, take the Malpensa Express train to Milan Central Station, then transfer to a train to Pavia. Taxis and car rental services are also available at both airports.

By Train:

Pavia is well connected to Milan and other major Italian cities through the national railway network. Trains from Milano Centrale or Milano Rogoredo reach Pavia in about 25–30 minutes.
You can find more information and train schedules at Trenitalia or Trenord.