To try to sum it up a BEC is used to allow you to use the main pack to power the motor and the servos. Whether internal or external.
Your other option is to run a straight LiFe or LiPo separately to power the servos.
This is where high voltage comes into play. HV servos can be run on any of the options listed however if you go with a low voltage (otherwise known as standard servos) then you will be unable to use a LiPo pack (unless you run a voltage regulator, which defeats the purpose in my opinion) but you could use a LiFe pack or if using a BEC The voltage will have to be dropped fairly significantly.
As Chris mentioned some HV servos are power hungry such as Savox or JR. (JR standard servos also eat through power) where as Futaba HV servos are not. Long story short, high voltage is in relation to what volts they can be powered by not how much they consume.
Hope that helps and doesn't confuse.
Regards,
Brandon Bartolomucci