I find a new tool: dvisvgm (https://github.com/mgieseki/dvisvgm)  ,which we can use it to preview latex fregments, it can deal with
xetex's xdv file as well (it will available in texlive2016).

The questions are:
1.  which path we should use:   "dvi/xdv -> svg" or "dvi/xdv -> svg -> png" ?
2. if we use the first path, how to deal with org-latex-preview-ltxpng-directory?  should we rename it to org-latex-preview-ltximg-directory or
   org-latex-preview-ltxpic-directory ?