I have found one famous person has the same name as you in Japanese Wikipedia.
"Povilas Vanagas", a Lithuanian ice dancer, is translated as 「ポヴィラス・ヴァナガス」 .
Japanese don't distinguish [l] and [r] , [b] and [v]. It might be difficult for your karate sensei to catch and distinguish the sounds of your name by their ear at first time.
If you accept Povilas as「ポヴィラス」to be spelt in Japanese, I assume one of the explanation to pronounce it like:
"Po" as "Pokémon" : 「ポケモン」
"vi" as "Venus" : 「ヴィーナス」
"las" as "Last" : 「ラスト」or "Las Vegas" 「ラスべガス」.
Use the bold part and concatenate altogether to pronounce your name. I hope it works.
I remember it was very difficult for me to pronounce "Woolworths", supermarkets in Australia. I pronounced "Wool" + "Worths" very slowly and tried to pronounce them at once. I recommend you ask him to pronounce your name very slowly at first.
There are questions discussed about the difficulties for Japanese to distinguish those sounds:
Do Japanese actually pronounce the "v" sound?
Why do Japanese speakers have difficulty pronouncing "L"?