Different Zener Diode Symbols
The base symbol I can fine no definite standard reference for.  I'd guess the majority of us would use it.





simulate this circuit &ndash; Schematic created using CircuitLab

Graphical samples for the Australian Standard AS 1102 (1989) Graphical symbols for electrotechnical does appear to reference this symbol.  I don't have a copy, so I cannot confirm this.

Europe: International Electrotechnical Commission IEC60617 (2012) Graphical Symbols for Diagrams provide the following:



with the IEC60617 (1996) version defining the bars.

for:


Schottky
Tunnel
Zener
Bi-directional
Unitunnel


Their usage of straight bars (05-02-04) is for Bi-directional or breakdown diode.



US: IEEE 315 (1975) Standard for Graphic Symbols for Electrical and Electronics Diagrams provides the straight bar symbol:



ANSI Y32.2 (1975) Graphic Symbols for Electrical and Electronics Diagrams has been superseded by IEEE315.  It references the same symbol as IEEE315.