Simply put they are polar opposites.
A positivist approach is one that is considered objective, detached and if I may put that way, structured by the researcher's views who tests a hypothesis. It is usually associated with quantitative data collection methods and statistical analysis.

A social constructivist approach is bottom up, situated within the group of phenomena you are researching, informed on an ongoing basis by the people and issues you are researching. It is usually qualitative and as such the researcher is part of the process. Objectivity is not the goal, instead you are looking for authenticity of findings making the voices of the participants central. Your goal is explaining a central phenomenon.

I will let someone else tackle the second part of your question.