As I understand, Istio VirtualService is kind of abstract thing, which tries to add an interface to the actual implementation like the service in Kubernetes or something similar in Consul.
When use Kubernetes as the underlying platform for Istio, is there any difference between Istio VirtualService and Kubernetes Service or are they the same?
Istio's VirtualServices provides, as every Istio's extensions, some additionals features such as external traffic routing/management (Pod to external communication, HTTPS external communication, routing, url rewriting...).
Take a look at this doc about it for more details : https://istio.io/docs/reference/config/networking/virtual-service
They can be both useful, as you need "classic" Services to manage ingress traffic or service-to-service communication.