We study the diffraction produced by a slab of purely reflective
PT-symmetric volume Bragg grating that combines modulations of
refractive index and gain/loss of the same periodicity with a quarter-period
shift between them. Such a complex grating has a directional coupling
between the different diffraction orders, which allows us to find an analytic
solution for the first three orders of the full Maxwell equations without
resorting to the paraxial approximation. This is important, because only
with the full equations can the boundary conditions, allowing for the
reflections, be properly implemented. Using our solution we analyze
unidirectional invisibility of such a grating in a wide variety of
configurations.