Kilauea was not formed by tectonic activity at a plate boundary. Instead, it is the result of a geologic hot spot beneath the middle of the Pacific plate. This hot spot also built the other islands of the Hawaiian chain, beginning with Kauai about 5 million years ago.
Hot spots occur where magma wells up through a crustal plate. This may happen because the plate has a weak or thin spot, or because the magma is particularly hot at that location. A similar hot spot lies under the American plate and is responsible for thermal features such as geysers, hot pools and mud pots at Yellowstone National Park.