Loss of life (also reffered to as "Loss of HP" & LoL) is an effect which reduces a champion's current HP. As is commonly stated, Loss of life is not considered to be damage, despite reducing HP & appearing similar in the interface. This means abilities such as Drive are separate from any ability/condition which mentions "damage". For example, loss of life does not get reduced by Impervious.
Loss of life will bypass all clauses that use the term damage as a trigger .
Deflect and Impervious are both abilities that use "damage" as a trigger for the abillity to take effect. Thus, LoL damage will not trigger these abilities.
Loss of life damage does not currently bypass defense. Defense currently reduces all sources of HP reduction, that lack a clause bypassing it. The exception to this is the ability Hex, which currently bypasses defense entirely, while lacking such a clause in it's description. This is considered to be a bug, and will likely change in the future.