What is the difference between postgres and postgresql_psycopg2 as a database engine for django?

It’s the same. django.db.backends.postgresql_psycopg2 used in django <1.8 and it was renamed in django 1.9 to django.db.backends.postgresql.
From docs:

Changed in Django 1.9:

The django.db.backends.postgresql backend is named
django.db.backends.postgresql_psycopg2 in older releases. For
backwards compatibility, the old name still works in newer versions.