The VIX is the more widely known index, representing the implied volatility of S&P 500 options that expire in 30 days. The VXV is the exact same index, except it represents IV for S&P 500 options that expire in three months. The VXV is usually higher than the VIX, but not always.
Using the weekly chart (click to enlarge) of $VXV:$VIX ratio (dashed line) superimposed with SPY price (black line), the ratio seems to bottom before the price most of the time. Note that the ratio (VIX:VXV) is inversed in the below chart.
As always, invest with your own research and risk. This article is just for your information.