It's a subspecialty that you practice after completing a fellowship (which requires you to have completed a residency program first). You can get into a palliative care fellowship from multiple specialties, including anesthesia, EM, FM, IM, OBGYN, peds, PM&R, psych, neuro, radiology, and surgery. Here's more info:
http://aahpm.org/uploads/AAHPM16_Medical_Student_BroWEB.pdf