Christmas is coming up soon, and most of us are so busy that they can only find time to use the term Xmas instead. First, it seems that only the form Xmas but not X-mas or xmas are acceptable. Moreover, Xmas can be pronounced either as [ˈkrɪsməs] (just like the word Christmas, note that the t is silent) or as [ˈeksməs]. You can listen to both variants here. In my experience, the pronunciation [ˈeksməs] seems to be quite rare, with [ˈkrɪsməs] being used instead. Hence, the use of the abbreviation Xmas is mostly used in written English. Finally, if you are a non-native speaker, make sure that you pronounce the X in Xmas in the appropriate English way (for example, German speakers often say [ˈiksməs]).
For a funny Xmas pronunciation story, see here.