It would be easy to do calligraphic letters as:
\mathcal{AABBCCQQZZ}
As 
But How can I get a calligraphic \pi or \Pi, I mean capital version of Pi.
I clearly remember that I saw calligraphic capital Pi somewhere.
It would be easy to do calligraphic letters as:
\mathcal{AABBCCQQZZ}
As 
But How can I get a calligraphic \pi or \Pi, I mean capital version of Pi.
I clearly remember that I saw calligraphic capital Pi somewhere.
The eulervm fonts are mildly calligraphic, but may serve your need. Here are a few letters in the font, including four pi' s at the end (lowercase, uppercase, and \mathbold [different than \mathbf]). Of course, once the package is loaded, all math symbols will be in this font.
\documentclass{article}
\usepackage{eulervm}
\begin{document}
$ABC\alpha\beta\gamma~\pi\Pi\mathbold{\pi\Pi}$
\end{document}

For comparison, here are the CM defaults:

EDIT: If I'm not mistaken, I seem to recall learning that the font designer Hermann Zapf designed the Euler math fonts to complement Palatino as the text font. Zapf is my favorite font designer, "the greatest who ever got ink on his coat" (hat tip to Tom Lehrer's song about N I Lobachevsky, http://www.youtube.com/watch?v=UQHaGhC7C2E&feature=kp).
\usepackage{eulervm}would be sufficient? – Steven B. Segletes Mar 26 '14 at 23:17