django content types – how to get model class of content type to create a instance?

You need to create an instance of the class. ct.model_class() returns the class, not an instance of it. Try the following:

>>> from django.contrib.contenttypes.models import ContentType
>>> ct = ContentType.objects.get(model="user")
>>> ct_class = ct.model_class()
>>> ct_instance = ct_class()
>>> ct_instance.username="hellow"
>>> ct_instance.save()