Regular Qt themes are compiled C++ programs that use the QStyle API to alter the look of Qt applications. They can do just about anything, but obviously require code to create. Being compiled programs also means they can't be portably distributed. They have to be recompiled for every different Qt version and architecture.

Kvantum is just one of those themes, and it uses its code to load and display much simpler SVG-based themes. Kvantum themes are actually much less complex than regular Qt themes, which is the whole point, since that makes them significantly simpler to create and much more portable, which is why they're so popular. The vast majority of Qt themes nowadays are made for Kvantum. Before Kvantum, it was mostly the less powerful QtCurve. Regular themes can do a lot of things Kvantum themes can't, but Kvantum is usually good enough.