I found mistake in my code.. I used button type = "button" as you can see
<aui:button type="button" value="save" onClick="showDialog()" />
but for using aui field validator work according to requirement, to prevent from submitting the form when required field is empty. use button type="submit". so corrected line is
<aui:button type="submit" value="save" onClick="showDialog()" />
Now its working fine :) :)