I recently experimented with Hyper-V 2012 and found there was no easy way to disable complex passwords, so after searching the net for a while I came across a nice solution that didn’t involve joining and leaving domains and interfering with group policy.  Here it is –
First step is to insert a USB stick and check which drive letter it is (D: in my case), then we can export our current policy:

secedit /export /cfg D:\new.cfg

Remove the USB stick and edit the file on another PC.  We are interested in PasswordComplexityvalue within the [System Access] section.

[System Access]
MinimumPasswordAge = 0
MaximumPasswordAge = 39
MinimumPasswordLength = 0
PasswordComplexity = 1
PasswordHistorySize = 0
...

Value should be changed to 0 in order to disable complexity check.

Save the file back, insert it back in to the Hyper-V server and import the configuration:

secedit /configure /db C:\Windows\security\new.sdb /cfg D:\new.cfg /areas SECURITYPOLICY

You can now add users with out complex passwords.

Créditos:http://sion-it.co.uk/tech/windows/disable-complex-password-requirement-on-hyper-v-server-2012/