[mrWiz]

Generally the fans are powered with a steady state DC source and the PWM is just logic signaling. If the fan is powered by a PWM source the tach output will not be a clean square wave (but could still be reconstructed, if the PWM frequency is sufficiently higher frequency than the fan's rotational speed).

[amelius]

Well, PC fans come in 3-wire and 4-wire forms, where the 3-wire type has only pwm,tach and GND connections (no DC power).

[robotguy]

3-wire is: TACH, DC, GND

4-wire is: PWM, TACH, DC, GND

https://noctua.at/en/what-pin-configuration-do-noctua-produc...

[amelius]

In practice pwm is used instead of DC.

[wtallis]

I can't remember the last time I encountered a motherboard that didn't offer you the choice between DC voltage and PWM for fan speed control. Both are still mainstream options; CPU heatsink fans are almost always 4-pin PWM fans but case fans are very commonly 3-pin DC fans. AIO Water cooling kits also commonly use 3-pin fans, especially when integrating their own fan controller or multiplexer so that they only need to occupy one or two fan headers on the motherboard.

(Edit: if you mean that in practice, the motherboard approximates DC voltage control using a PWM-based method, that may be true, but it's beside the point; a 3-pin fan is missing the control circuitry that a 4-pin PWM fan uses, and the 3-pin fan's behaviors like turn-on point will be specified in terms of DC voltage not PWM duty cycle, and you can't drive a 3-pin fan with the PWM signal intended for a 4-pin fan because the voltage and current supplied are both far too low.)