What is the difference between Al-Kitāb and Al-Qur'ān, when both are from Allah Almighty?

There is no difference between "the Book" mentioned in the opening of Surat Al Imran and the Qur’an. What is meant by it is the Qur’an. The Qur’an has been called "the Book" in many surahs, just as it has been called "the Criterion" (Al-Furqan).