Different between SNMP v1, SNMP v2c and SNMP v3?

SNMPv1: 

Five operations/PDU: GET-RESPONSE, GET-NEXT, GET-request, SET, TRAP.

Security: Community based. (No authentication no privacy)

SNMP Community strings are not encrypted(clear-text) in v1 & v2c. Community string will be easily identified by packet sniffer.

SNMPv2c: 

Two new operations/PDU – GET-BULK(efficient retrieval of multiple OIDs), informs (trap + ACK mech.), plus 64-bit object support.

Security: Community based. (No authentication no privacy)

SNMP Community strings are not encrypted(clear-text) in v1 & v2c. Community string will be easily identified by packet sniffer.

 SNMPv3:

PDUs: Includes all the v1&v2c  pdu's.

Security: New architecture including security functionality; authentication (SHA, MD5), privacy (encryption - DES, 3DES, AES). Uses Authentication(MD5 or SHA). Encryption using CBC-DES(Adds DES 56-Bit Encryption in Addition to Authentication Based on DES-56).