I always figured hollow was a misnomer, earth is more of a honey comb network of very large "caves".
Molten core, Geo-thermal provides heats, luminescents to provide light.
Volcanoes are very real, so yea it has to come from somewhere.
My point is, "hollow" earth doesn't have to exclude molten core. I'm not saying it does exist either, but I don't think it's really all that far fetched either.