  We present a correspondence between positive operator valued measures (POVMs)
and sets of generalized coherent states. Positive operator valued measures
describe quantum observables and, similarly to quantum states, also quantum
observables can be mixed. We show how the formalism of generalized coherent
states leads to a useful characterization of extremal POVMs. We prove that
covariant phase space observables related to squeezed states are extremal,
while the ones related to number states are not extremal.
