Symantec's ApplicationHA only monitors the health state of the app, and then restarts the VM if it fails. That will achieve HA, but not zero downtime.
Neither will a FT solution like Marathon or VMware FT, they run a single OS in lock-step across 2 machines... which provides zero downtime for just a very narrow set of uncommon scenarios... like if your server has a hard complete failure. But if you have a software failure, they do nothing.
As stated above, it will likely take a combination of technologies to achieve the best solution. And the less downtime a solution delivers, the higher the price tag will be. So while naive managers like to state lofty goals of zero downtime, when they see the price tag... they usually think twice. Beyond the intial cost, you need to ensure you have the IT staff to maintain a really complex system... there are many variables. So you need to establish the SLA's up front.
