There's something odd about your data. The website I consulted gave $c_v$, the molar heat capacity at constant volume as 12.4717 $\text J\ \text{mol}^{-1}\ \text {K}^{-1}$ for both helium and neon (I couldn't find Xenon). Both of these were measured at 25 °C, though for inert gases there is very little change of $c_v$ with temperature.
This value of $c_v$ agrees to five sig figs with the theoretical value of $\frac32 R$.
So at equal temperatures the same amount (number of moles) of the gases have the same internal energy, given by $$U=\tfrac32 nRT.$$ This works for all monatomic gases at lowish densities (so they behave as ideal gases).