It can be either positive or constructive. In order for feedback to be effective it must be descriptive, appropriate, well-timed, owned, and positive. Constructive feedback serves a corrective function in that the other person's behavior is described in terms of its negative impact or affect on others.
Positive feedback is both affirming and reinforcing.
Over the course of the next week, look for opportunities to give each individual you mentor positive feedback. Watch to see when the individuals that you mentor do the behaviors that you have identified as desirable. Plan out what you are going to say as well as when and how you are going to communicate positive feedback. How Think about how you are going to communicate the feedback. In a written thank-you note Give the other person the feedback.