I haven't heard of IV mitts, but I have seen patterns for 'PICC' bands. (and 'cast socks'.. to go over a person's toes while their foot is in a cast)
Making a mitt with a thumb hole (etc) with the idea that it;'ll fit over someone's IV (in their wrist/hand) would be problematic... IV's are placed in too many places (and they vary by individual too). Plus (speaking as an RN) they would probably get in the way (and if someone is sick enough to need an IV, easy, immediate access to it is important).
PICC lines, on the other hand, are being used a lot more both for in-hospital use and for long-term-care, and even home infusion (PICC lines-peripherally inserted central catheters, for those who don't know- are like really long IV lines inserted into the upper/inner arm) and having a band over them really helps to keep them out of the way (and since they are just a short sleeve, they can be easily moved out of the way). They are much more 'stable', and kept in place for (up to) a month at a time (unlike an IV which needs to be changed every couple of days, and has a greater tendency to 'clog' or slip out of place).