Conduction and convection are forms of heat propagation that require a material medium to occur, however, there is a form of heat propagation that does not require a material medium (vacuum) to propagate, this is the thermal irradiation.
This type of heat propagation occurs through infrared rays that are called electromagnetic waves. This is how the Sun heats up the Earth every day, and it is also how the thermos keeps the coffee hot inside for a long time.