The Visa, MasterCard and Discover Card Verification Number is a 3-digit number that is printed on the back of your card. It typically appears to the right of your card number.
CVN location on VISA, Mastercard and Discover.

The American Express Card Verification Number is a 4-digit number printed on the front of the card. It typically appears to the right of your card number.
CVN location on American Express.