Power refers to the objective aspects of your reality that are outside your control, but have power over you. Example: Laws of Physics (Gravity), Limitations (Human Physical Limits), Laws of the Country (Police, Military), etc. These limit your reality and you have no choice in the matter.
Principle refers to limitations you subjectively choose to impose on yourself, because you believe in them. For example, you choose to make 11PM your bedtime because you want to have the benefits of a consistent sleep schedule for good health, and you want to start your day early (How to Become a Morning Person).
Preference refers to your subjective choices and feelings that make up your personal reality. Principles are a subset of this reality.