Kīlauea is located in the middle of the Pacific Plate, unlike other volcanoes formed along a plate boundary, Kīlauea forms over a hot spot. This means the magma flows through oceanic crust, directly from the mantle, erupting basaltic magma.
Kīlauea's magma comes from partial melting of the mantle, specifically ultramafic mantle rock: peridotite. Because of the mafic setting, this means the magma is low in silica, rich in iron & magnesium, and low in dissolved gas, which produces basaltic magma. This causes effusive eruptions which are non explosive, gentle lava flows.