Pluto's internal heat source

Radioactivity.

Best supported, as pointed out by Emilio Pisanty, on §2.2 of Icarus 216, 426 (2011) (eprint).

First, Pluto having an internal source of heat is indeed the most likely scenario (if Pluto and Charon are as old as expected), not only given its apparent youth, but also due to the absence of compressional features on its surface, according to recent paper,

• Recent tectonic activity on Pluto driven by phase changes in the ice shell. N.P. Hammond et al. Geophys. Res. Lett. 43, 6775 (2016).

which Ars Technica nicely summarizes here.

The same paper indicates through simulations that radioactive heating is a viable scenario:

The core is heated by long lived radioisotopes $^{238}$U, $^{235}$U, $^{232}$Th and $^{40}$K. We assume an initial abundance of parent isotopes appropriate for CI chondrites.

Also, according to this old NewScientist article:

If the core contains potassium at a concentration of 75 parts per billion, its decay could produce enough heat to melt some of the overlying ice, which is made of a mixture of nitrogen and water. It should have at least that much potassium and probably more, says William McKinnon at Washington University in St Louis, Missouri.

The same article points out that the existence of an ocean today depends on "the sloshiness of the ice that covers it" as well, which should be strong enough to decrease convection and, thus, loss of heat to space.