In quantum field theory in flat spacetime, there are both positive and negative frequency solutions to the classical field equations, but upon quantization we get only positive energy particles. But in Hawking's original paper about Hawking radiation, it is stated that negative energy particles can exist inside a black hole:

Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy. The negative particle is in a region which is classically forbidden but it can tunnel through the event horizon to the region inside the black hole where the Killing vector which represents time translations is spacelike. In this region the particle can exist as a real particle with a timelike momentum vector even though its energy relative to infinity as measured by the time translation Killing vector is negative. The other particle of the pair, having a positive energy, can escape to infinity where it constitutes a part of the thermal emission

That is, Hawking is saying that the energy of a particle can be defined as $$E = p^\mu K_\mu$$ where $p^\mu$ is its four-momentum and $K^\mu$ is the time translation Killing vector. I can see how this works in Minkowski space, where $K = \partial_t$ and we get $E = p^0$ as expected.

But why is this the correct definition of energy? What kind of observer would measure $E$ to be the energy of the particle? Can this quantity be shown to be conserved? Why should we trust this equation when $K^\mu$ is not even timelike inside the black hole?