XP is awarded when you deliver passengers and cargo to a destination. The amount of XP given corresponds with this formula:
XP= C+P+(500* B)
B- Total amount of bux for items delivered
C- Total amount of coins for the cargo delivered
P- Total amount of coins for passengers delivered
XP is either shown on the bottom or in the settings tab of the menu
Flying Coins XP
You can recive XP for the coins that fly past the plane. You cannot recive XP for the bux, however.
Getting More XP
In order to get more XP you need to deliver more items paid in bux. This is because there is a bonus upon bux items. Also a slow but easy way to get more, is to tap all the coins that pass the plane.