CVV number is generally last 3 digits on the reverse side of the card where signature block is made. It is an added layer of security to ensure 100% safe and secure online transactions. This is valid for Visa card, Master Card & Discover Network cards.
Please have a look at below screen shot. This number is printed on the signature panel on the back of your card, after the card number. The CVV number has been indicated in adjacent screen shot with red arrow.