The text on the left hand side is not displaced but written with white color. The reason is a color stack problem resulting from vsplitting. pdflatex and xelatex seem to have a different color implementation.
I changed the placing of text colors for tcolorbox with the recent version 3.12 (2014/07/29).
This version was tested with MiKTeX. The following test file compiled without problems with different engines:
\documentclass{article}
\usepackage{lipsum}
\usepackage{tcolorbox}
\usepackage{paracol}
\tcbuselibrary{breakable,skins}
\begin{document}
\begin{paracol}{2}
\begin{rightcolumn}
\lipsum[1-2]
\begin{tcolorbox}[breakable,enhanced jigsaw,
size=small,colback=red!10!white,colupper=blue!50!black,
vfill before first,height fixed for=middle,
pad at break=1mm]
\lipsum[1]
\end{tcolorbox}
\end{rightcolumn}
\begin{leftcolumn}
\lipsum
\end{leftcolumn}
\end{paracol}
\end{document}
I compiled with three engines:
This is XeTeX, Version 3.1415926-2.5-0.9999.3 (MiKTeX 2.9) (preloaded format=xelatex 2014.6.23)
and
This is pdfTeX, Version 3.1415926-2.5-1.40.14 (MiKTeX 2.9) (preloaded format=pdflatex 2013.6.27)
and
This is LuaTeX, Version beta-0.76.0-2013062820 (rev 4627) (format=lualatex 2013.12.20)
All three produced the same output:

Note that it is still possible to provoke color errors in combination with paracol, if color commands are written into the box text. These errors appear for tcolorbox and mdframed alike. If this happens, it seems to be enough to place a \color{.} inside the column where colors disappear or are set wrong. This also worked in my experiment for tcolorbox and mdframed alike. If no color commands are written into the box text, I found no problems with the development version using MiKTeX.
tcolorboxwithout problems. I compiled withpdflatex. – Thomas F. Sturm Jul 26 '14 at 08:50