give resons for

KCN reacts with an alkyl halide in a direct S_N reaction to give the product. So it could be S_N2 or S_N1.

AgCN is special, in that Ag has a special high affinity for halogens. It will automatically abstract the halogen and form a precipitate, leaving a carbocation and a CN^- ion. The reaction path followed is always S_N1 because the formation of precipitate shifts equilibrium of formation far forward. The carbocation formed is irrespective of its stability.
As per Hard Soft Acid Base theory, a hard acid carbocation will like to bond with a hard base nitrogen. Thus resonance in CN anion takes place and NC^- attacks the carbocation, forming an isocyanide.
Thus a cyanide ion is called an ambident nucleophile, as it can attack from more than one nucleophilic centers.