I would say "go with PV" (on Linux) unless you've got a reason. PV has been far better tested because it had been the sole choice for several years. (HVM for UNIX was solely recently introduced outside of the terribly special calculate Cluster nodes.)
Converting between them isn't hard -- particularly if you are doing it right. By "do it right", I mean using something like Chef or Puppet or Ansible to create your server. This enables you to put your "server" (well, server build process) underneath version control, that creates instant documentation for everyone to understand what is on your server.
Your root drive should be ephemeral so that if the server dies, you can just re-launch and re-attach your EBS data. (And you'll be able to quickly upgrade the OS or server software by launching a second instance to play with.) If you've got an EBS root drive, you'll have to pay to store/backup your OS, and you may never know if your server config is checked into version control or not.
Going forward, I might lean toward HVM, since performance can be far better (i.e. "Enhanced Networking"). Also, it's not "brand-new" anymore.
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