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What is the practical difference between Linear and Non Linear Modulation Techniques ?

M ary Psk and M ary Qam are examples of linear modulation, However they are different from each other in such a way that M ary Psk signal has a constant envelope, whereas M ary Qam involves changes in the carrier amplitude.

I am also confused as how PSK is said to be linearly modulated technique ?

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In PSK the phase is modulated. The linear bit probably comes from the use of linear operators. – geometrikal Nov 24 '12 at 12:53
What exactly do you mean by "linear modulation"? The defining property of linearity is superposition, which doesn't seem to apply to a digital scheme. – Jim Clay Nov 24 '12 at 14:38
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$4$-PSK = QPSK = $4$-QAM is about all that one could consider to be linear modulation (of two phase-orthogonal carriers) in any sense of the word "linear". More generally, as @JimClay says, for digital schemes, the concept of linear modulaton makes little sense. For example, $M$-QAM is really two $\sqrt{M}$-PAM signals modulated on phase-orthogonal carriers and for $M > 4$ we get into how bits are interpreted as levels, whether data has been Gray-encoded or not, etc. – Dilip Sarwate Nov 24 '12 at 20:20

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