What improvement was made in Su-35 over Su-30?
The Su-30 is an modifications of the SU-27UB
The Su-35 is an Su-27 shaped aircraft with many new parts from the Su-34 (T-10BM)
in detail
Radar N035 Irbis-E PESA
Rear-looking self-defense radar
new fly-by-wire flight-control system
new fully-glass cockpit
Electronic-warfare self-defense system L175M Khibiny-M
Engine AL-37FU/ 117S
larger Air Intakes, no canards, reduced-area empennage  

Su-35 has been also called early export versions of the Su-27
and to make more confusion, the production factories (Irkut / Amur) does not name their product Su-35  

Please take a look at   http://www.sukhoi.org/eng/planes/military/Su-35/
and   https://defence.pk/pdf/threads/su-30mki-vs-su-35s.222304/