Deviance is behavior that is contrary to the norms and values of a society. Positive deviance is behavior that is outside the norms and values but is nevertheless good. It is hard to objectively determine what behaviors are examples of positive deviance until after the fact.
For example, the actions of Dr. Martin Luther King, Jr. would now be seen as positive deviance. His actions were clearly opposed to the prevailing values of the society within which he lived. However, they were clearly (from our modern point of view) very positive. They challenged the values of the time and were therefore deviant, but the values they challenged were bad values. Therefore, these behaviors were examples of positive deviance.