Constructionism is a theory describing that learning is only effective when the learner is actually learning actively. It says, that all knowledge is already existent. The learner has to construct new knowledge, meaning subjectively new, with learning-by-doing. The goal is to master the new complex situation by cooperating, communicating and interacting. The teacher in constructionism is in a quite passive position and understood as a coach or trainer.
Behaviourism describes the process of learning by stimulating the learner. A stimulus can create certain behaviour with the learner. By repeating the stimulus the learner gets conditioned to react in a certain way. This reinforcement of behaviour can be used in different ways to manifest behaviour. For example: if one wants to strengthen a certain behaviour, a positive reinforcement (like a treat or so) will produce a pleasant consequence for the learner. In order to get a treat again, the learner will repeat the behaviour.