Here's one way of looking at the two concepts.
Non-violence is clearly most impressive when you can put a million people on the streets. The best examples, according to Gene Sharp, are those that are planned and run by a committed, knowledgeable cadre of leaders. In effect, we see a struggle between two groups, the resistance leadership and the regime, backed by (it's hoped) nonviolent protesters on the one side, and police and army on the other.
Defense, on the other hand, is individualistic. Criminologist Gary Kleck for example has noted that Americans use firearms for defense on the order of 2.5 million times per year, and annually kill twice as many criminals as police do. These are virtually all individual actions.
Given the different venues for the two, it may be that one does not necessarily preclude the other.
It's clear that with large actions such as the recent overthrow of Mubarak in Egypt, you want to keep the protesters nonviolent, and guard against regime provocateurs turning it violent. But when you have come home from the demonstration, and someone pounds on your door at 3 AM in the morning to arrest you, what end does maintaining a nonviolent posture serve? Keep in mind the Egyptian action included informal block defense committees arising spontaneously as police activities vanished, and many of these included armed defense. In what way did this harm the overall non-violent protest? Could it not actually have helped it, instead, by showing people that protection can be had without police? Didn't it thus further reduce the legitimacy of the police, a usual aim of nonviolent protest?