  Cosmic variance is the intrinsic scatter in the number density of galaxies
due to fluctuations in the large-scale dark matter density field. In this work,
we present a simple analytic model of cosmic variance in the high redshift
Universe ($z\sim5-15$). We assume that galaxies grow according to the evolution
of the halo mass function, which we allow to vary with large-scale environment.
Our model produces a reasonable match to the observed ultraviolet luminosity
functions in this era by regulating star formation through stellar feedback and
assuming that the UV luminosity function is dominated by recent star formation.
We find that cosmic variance in the UVLF is dominated by the variance in the
underlying dark matter halo population, and not by differences in halo
accretion or the specifics of our stellar feedback model. We also find that
cosmic variance dominates over Poisson noise for future high-$z$ surveys except
for the brightest sources or at very high redshifts ($z \gtrsim 12$). We
provide a linear approximation of cosmic variance for a variety of redshifts,
magnitudes, and survey areas through the public Python package galcv. Finally,
we introduce a new method for incorporating priors on cosmic variance into
estimates of the galaxy luminosity function and demonstrate that it
significantly improves constraints on that important observable.
