To navigate these difficult conceptual waters rules are needed. Here are three suggestions (the violence can be direct--as sometimes prescribed by the Abrahamic religions--or structural as by Hinduism):
 Anchor "religious fundamentalism" in religious texts taken literally, according to the fundamentalists; not as "interpreted";
 Anchor "extremism" in violent action, verbal or physical;
 Anchor "religious extremism" in violent action justified-legitimized by religious texts, by fundamentalists or not.
Fundamentalism has to do with inner faith-belief. Extremism has to do with outer violence against Other and against Self (like flagellation for being a sinner). Keep them separate. And be careful.
We can have fundamentalism without extremism. The fundamentalist may believe much beyond the beliefs of others yet not cross the border to violence. We might say: let him-her do so; it is far from obvious that fundamentalists are more violent than non-fundamentalists.
We can have extremism without fundamentalism. Most people exercising violence believe in nothing beyond "doing their job".
There are two criteria for "religious extremism": violence and religious legitimation. That legitimation may be fundamentalist or not; could be just well-know quotes from the Scriptures. We might even speculate that for the fundamentalist faith may be sufficient.
The combination in "religious extremism" is vicious if it implies that violence will be supported by divine forces and-or that failure to be violent will incur their wrath. Probably a declining category.
Today's secularizing, "enlightened" world brought us statism, nationalism, and their combination; secular fundamentalists and extremists, and their combination. They have given the world more violence for victory for whatever cause they design than religions. But with a rationality that may open for solving underlying conflicts.