Explanation/Reference:
Explanation:
SNMPv1, SNMPv2c, and SNMPv3 each represent a different security model. The security model combines with the selected security level to determine the security mechanism applied when the SNMP message is processed.
The security level determines the privileges required to view the message associated with an SNMP trap.
The privilege level determines whether the message needs to be protected from disclosure or authenticated. The supported security level depends upon which security model is implemented. SNMP security levels support one or more of the following privileges:
noAuthNoPriv-No authentication or encryption

authNoPriv-Authentication but no encryption

authPriv-Authentication and encryption

SNMPv3 provides for both security models and security levels. A security model is an authentication strategy that is set up for a user and the role in which the user resides. A security level is the permitted level of security within a security model. A combination of a security model and a security level determines which security mechanism is employed when handling an SNMP packet.
Reference: http://www.cisco.com/en/US/