In a vacuum, heat is transferred by radiation.
Thermal radiation occurs through a vacuum or any transparent medium (solid or fluid, or gas). In a vacuum, heat transfer occurs through the radiation mode of heat transfer. In a vacuum, heat can’t be transferred by conduction or convection. The process of convection and conduction requires some medium made of material particle for transmission of heat. In a vacuum, there is no material. So, heat travels in a vacuum by radiation.
In radiation, heat is transferred in electromagnetic radiation, which requires no medium to propagate. In conduction, heat is transferred from particle to particle. There are no particles to transfer the heat in a vacuum, so conduction cannot transfer heat in a vacuum. In convection, heat is transferred by the actual movement of particles, but a vacuum has no particles, so convection cannot work in a vacuum.