difference between linear modulation and non-linear modulation scheme?
According to this source:

Digital modulation techniques are classified as linear if:


The amplitude of the transmitted signal varies linearly with the modulating digital signal, \$m(t)\$.
They usually do not have a constant envelope
They are more spectral efficient.
Poor power efficiency


The only other "linearity"-reference I could find was in the same course slides:

For Frequency Modulation, the relationship between received power and quality is non-linear (Rapid increase in quality for an increase in received power).

For Amplitude Modulation there is a linear relationship between the received signal quality and received signal power.

But I am not totally convinced that this is at the basis of the definition (I'm also not totally unconvinced).

IMHO, I think they started out calling the AM scheme linear, as \$AM(a\cdot V_{in}) = a\cdot AM(V_{in})\$, and then they started playing with the phase ((Q)PSK) and just labeled it as linear as well, while it isn't strictly true.

[EDIT] I also found this which seems to support my last remark.