By Plane: 

One can fly to one of Milan's 3 airports: Malpensa (90km from Pavia), Linate (50km from Pavia) or Orio al Serio (70km from Pavia).

Malpensa Airport (MXP)
There are several buses and trains connecting the airport to the Milan’s central railway station. From there, you can take a train to Pavia ("Pavia" Station).

Milan Linate Airport (LIN) & Orio Al Serio International Airport (BGY)
Frequent regular buses link both airports with Milan’s central railway station. Once there, take a train to Pavia ("Pavia" Station).

Regardless the airport you are arriving at, the journey from Milan’s central railway station to Pavia takes around 45min. with a short stop over. 

By train:

Many European cities are very well connected to Milan by direct train.

Milano Centrale station is the main one, but some other like Milan Rogoredo, Milan Bovisa Politecnico and Milan Porta Garibaldi are also connected to Pavia through regional direct trains.