I think I would describe the character of the ever-present “background” fatigue in ME/CFS as being most like the fatigue you feel when you’re initially coming down with the flu. It’s not PEM, because it’s a constant regardless of exertion. Fatigue like this can be the very first sign of the flu, occurring even before you develop a fever.
I don’t know if PEM is just an amplification of this “background” fatigue (which can itself be disabling) or if PEM is something else that lies on top of the “background” fatigue. In my experience, PEM usually starts the day after exertion and diminishes over time (so long as you avoid further exertion), but the background fatigue is always there, whether you exert yourself or not.
I’d also add that I think there’s more to this ever-present feeling than just “fatigue.” There’s a sense of “interference” with normal function, both physically and cognitively. It’s like you’ve consumed something that’s producing some kind of side effects – like headache, sweating, chills, OI, tachycardia, light sensitivity, memory problems and so on.