\documentclass[12pt]{standalone}

\usepackage{ucs}
\usepackage[utf8x]{inputenc}

\usepackage[T1]{fontenc}
\usepackage{textcomp}
\renewcommand{\rmdefault}{ugm}
\renewcommand{\sfdefault}{phv}
\usepackage[garamond]{mathdesign}
\usepackage{tikz}
\usetikzlibrary{positioning, shapes, arrows, shadows, }

\begin{document}

\tikzset{>=latex}
\begin{tikzpicture}[
  observed/.style={circle, draw},
  unobserved/.style={draw, circle, fill=gray!40},
  legend/.style={rectangle, draw},
  partly observed/.style 2 args={draw, fill=#2, path picture={
      \fill[#1, sharp corners] (path picture bounding box.south west) -|
      (path picture bounding box.north east) -- cycle;},
     circle}
  ]


  \node[observed] (y) {Y};
%   \node[observed,above=of y, xshift=1cm] (r) {R};
  \node[partly observed={white}{gray!40}, above=of y] (x) {X};
  \node[observed, left=of x,xshift=-1] (w) {W};
  \node[unobserved, above=of w, xshift=1cm] (k) {K};
  
  \matrix [draw, below, yshift=-0.2cm, font=\small, align=center, column sep=2\pgflinewidth, inner sep=0.6em, outer sep=0em, nodes={align=center, anchor=center}] at (current bounding box.south){
    \node[observed,label=right:observed] {}; \\
    \node[unobserved,label=right:unobserved]{}; \\
  };

  \draw[->] (x) -- (y);
  \draw[->] (x) -- (w);
  \draw[-] (k) -- (x);
  \draw[->] (k) -- (w);

\end{tikzpicture}
\end{document}

