Actually its plasma, not lava. Plasma is a superheated gas, in this case mostly hydrogen but with some other elements also present. The sun is composed of plasma right the way though to the core, though it gets thicker and more pressurized the closer to the core you get.
It's not "burning" in the sense we are most familiar with (chemical combustion), but the energy that keeps it hot is generated by nuclear reactions in the core that can occur due to the high temperature and density.
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