Virtualized Network I/O (VNIO) plays a key role in providing the network connectivity to cloud services, as it delivers packets for Virtual Machines (VMs). Existing paravirtualized solutions accelerate the virtual Switch (vSwitch) data transfer via memory-sharing mechanism, that unfortunately impairs the memory isolation barrier among VMs. In this paper, we categorize existing para-virtualized solutions into two types: VM to vSwitch (V2S) and vSwitch to VM (S2V), according to the memory-sharing strategy. We then analyze their individual VM isolation issues, that is, a malicious VM may access other ones' data by exploiting the shared memory. To solve this issue, we propose a new S2H memory sharing scheme, which shares the I/O memory from vSwitch to Hypervisor. The S2H scheme can guarantee both VM isolation and network performance as the hypervisor acts as a "setter" between VM and vSwitch for packet delivery. To show that S2H can be implemented easily and efficiently, we implement the prototype based on the defacto para-virtualization standard vHost-User solution. Extensive experimental results show that S2H not only guarantees the isolation but also holds the comparable throughput with the same CPU cores configured, when comparing with the native vHost-User solution.