It isn't unusual for VIX derivatives to follow their own path for short periods of time, as they aren't perfectly correlated with the market indexes. This is especially true when VIX levels are at extreme lows or highs.
Exactly lindq. That's the beauty of these products, you can even make money in a down market with it. Very profitable if you're on the right side of the trade and that's always short VXX or long XIV.
It looks like it closed below the NAV on friday by a decent amount. Odds are the majority of the gain you are seeing today is just due to the price catching up to the NAV since it closed below. http://www.velocitysharesetns.com/xiv
XIV has negative convexity vs. the SPX, meaning that it does better (worse) than a linear SPX beta model would predict on days with small (large) squared SPX returns. In other words, XIV tends to outperform when the absolute value of the SPX move is small.