If my Hot Pizza is in deep space and there is no gas molecules around it, how would it dissipate it's energy? There are neither conduction nor conviction, and since the Pizza is solid it can't radiate its heat will it stay hot?
It will still radiate heat in the infrared part of spectrum. It doesn't matter whether it is solid or not. See Black-body radiation for details.
Heat may be considered to be movement of particles. The particles of your hot pizza oscillate. An accelerating electric charge creates an electromagnetic field. The oscillating particles of the pizza emit photons in the wavelength of the field the particles create, generally infrared radiation. As the particles on the surface of the pizza radiate these photons, they become less energetic. For a while, conduction from hot particles within the pizza keep the surface particles oscillating. But eventually, all the paricles of the pizza become less energetic, their heat vibration slows, and the pizza cools.
See the first answer to this similar question: https://space.stackexchange.com/questions/7830/how-is-heat-in-space-dissipated.
Were heat never radiated away from the pizza as photons of electromagnetic radiation, the pizza would remain hot forever. Such a state would contravene the increasing entropy of our universe.