Assuming you are referring to milk kefir here. Please state if otherwise. (water kefir). Not that my answer would be any different.
I choose to use xylitol. Others, including Hv, have suggested that it will kill the beneficial bacteria/probiotic strains. HOWEVER, there are numerous probiotics on the market (chewable) which include xylitol in their formulas. Here's one:
I have written to NOW and was informed that their studies have shown that it does not affect that particular strain.
You do have other options. Chicory is one, erythritol another. Check out the Whole Approach websites for other options.
I am personally convinced that I am okay with xylitol. But to mitigate any destruction of friendly flora I hit it hard with probiotics, SBO's, and probiotics.