Override your form's clean
method:
class MyRegistrationForm(forms.Form):
password1=...
password2=...
...
def clean(self):
cleaned_data = self.cleaned_data # individual field's clean methods have already been called
password1 = cleaned_data.get("password1")
password2 = cleaned_data.get("password2")
if password1 != password2:
raise forms.ValidationError("Passwords must be identical.")
return cleaned_data
See the docs for more info.
You should also probably ALSO add some Javascript to check this on the client side - client side validation is no subsitute for server-side validation, but it is more responsive to the user and saves bandwidth.