Tawba is from Taaba (تاب , يتوب) means return to Allah with repentance on the bad deeds committed with decision not to do it again.
Istighfaar is seeking forgiveness. Tawba can include Istighfaar as it is wider that just the Istighfaar.
In Quran we read (إستغفروا ربكم ثم توبوا إليه ) Seek forgiveness from your Lord and repent (return) to Him (Hood 52). There is no Tawba with out Istighfaar.