PvE stands for Player versus Environment. It comprises the game modes where the player fights against the aliens, and includes survival modes and boss fights. Playing offline eliminates much lag, while playing online results in stronger enemies and higher amounts of gold being dropped depending on how many teammates are in the match. Playing offline in single player mode allows you to see how many experience points you are earning after each fight. When playing online, the results screen is modified to show each teammate's earnings in a variety of categories, including alien kills, amount of gold earned from the boss and gold/energy picked up. As a result, one cannot see how much EXP they earn from online PvE matches, though it is presumably much higher than their offline counterparts.