...and despite the fact it says it only applies to SEP11 and SEP12.0, the below link is the best article I can find regarding support for installing anything on a hypervisor/host machine (Hyper-V):
http://www.symantec.com/docs/TECH106269
I can find no articles to refute this, nor any that define compatibility with sticking a SEP client on ESX(i).
I would go with "No you can't install a SEP client directly onto your Hypervisor".
That's not to say I don't recommend you use SEP for your virtual guests though. As has been linked by Mithun above, SEP12.1 includes several technologies for reducing overhead on virtual platforms without compromising security.
Further to the VIE tool mentioned by Mithun, there's also the Shared Insight Cache: http://www.symantec.com/docs/HOWTO55311
Finally, just to pre-empt a usual question that follows, SEP12.1 does not use vShield (article lists all the overhead reducing technologies in SEP12.1):
http://www.symantec.com/docs/TECH175568
While not applicable in your case, it's also great that these technologies are available regardless of what virtual platform is used.