We propose a new class of Photonic Crystal cavity - namely, nanowire array based band edge and band gap PC cavities. This concept combines the advantages of PCs for tight confinement of light, with nanowires demonstrated suitability for fabricating lasers.A model is developed and used to evaluate the optical properties of band edge cavities. Given appropriate conditions, the exact treatment can be reduced to simple formulae, which provide an excellent approximation for field patterns and frequency values. We find boundary orientations, for which the lifetime of the confined modes can be enhanced by orders of magnitude. In band gap and "PC-based heterostructures" cavities, energy confinement is even more efficient. An optical mode with a small volume, on the order of (lambda/n)3, can have quality factor (Q) as high as 4 x 104. The formulae and simulation results provide guidelines for designing a compact and tightly confined optical mode in these cavities.