What is the minimum frequency of AC source to show IV curve on oscilloscope?
The frequency does not have to be very high. For every Hertz the complete curve gets drawn 1 time per second. If you use a 50 Hz source, the curve will be drawn 50 times per second. This should be a good value to get flicker free pictures.
If you use a lower frequency you will see the dot wander around the screen slowly. It will still draw the curve, but because the glow of the screen (of an analog scope) is not long enough to capture the complete draw, you always will only see parts of that curve. But already something like 5 Hz should be sufficient to see the complete curve, it just might be quite a bit flickery. With a digital scope it might depend a little bit on the type of scope, but typically they try to imitate the analog effect.