All history books simply call him Conrad. Everyone else who has a von X name is called by the name that comes after the von. Von Bismarck is called Bismarck. Von Moltke is called Moltke. Von Schlieffen is called Schliefen.
Von Hötzendorf is called...Conrad.
Why?
The best I know is that Conrad is in fact his family name (given name being Franz). The Hötzendorf name came from his grandmother's family. His primary use of the name Conrad instead of Hötzendorf probably has something to do with his male lineage not actually being part of the Hötzendorf noble family.