  We performed 3D hydrodynamic simulations of the inner $\approx 50\%$ radial
extent of a $25\ \mathrm{M_\odot}$ star in the early phase of the main sequence
and investigate core convection and internal gravity waves in the core-envelope
boundary region. Simulations for different grid resolutions and driving
luminosities establish scaling relations to constrain models of mixing for 1D
applications. As in previous works, the turbulent mass entrainment rate
extrapolated to nominal heating is unrealistically high ($1.58\times 10^{-4}\
\mathrm{M_\odot/yr}$), which is discussed in terms of the non-equilibrium
response of the simulations to the initial stratification. We measure
quantitatively the effect of mixing due to internal gravity waves excited by
core convection interacting with the boundary in our simulations. The wave
power spectral density as a function of frequency and wavelength agrees well
with the GYRE eigenmode predictions based on the 1D spherically averaged radial
profile. A diffusion coefficient profile that reproduces the spherically
averaged abundance distribution evolution is determined for each simulation.
Through a combination of eigenmode analysis and scaling relations it is shown
that in the $N^2$-peak region, mixing is due to internal gravity waves and
follows the scaling relation $D_\mathrm{IGW-hydro} \propto L^{4/3}$ over a
$\gtrapprox 2\ \mathrm{dex}$ range of heating factors. Different extrapolations
of the mixing efficiency down to nominal heating are discussed. If internal
gravity wave mixing is due to thermally-enhanced shear mixing, an upper limit
is $D_\mathrm{IGW} \lessapprox 2$ to $3\times 10^4\ \mathrm{cm^2/s}$ at nominal
heating in the $N^2$-peak region above the convective core.
