The Android Virtualization Framework enables the execution of security-sensitive workloads in protected virtual machines using trusted hypervisors. We present Aster, an in-depth analysis of the Android Virtualization Framework security model as defined in the Android Compatibility Definition Document. It explores the design space for deploying protected virtual machines across Arm Trusted Execution Environments. Our analysis shows that executing Android in the normal world and protected virtual machines in the realm world using Arm Confidential Computing Architecture achieves the best tradeoff between security and implementation overheads. Aster strengthens Android Virtualization Framework isolation guarantees by introducing improved memory protection to mitigate physical attacks, enhancing independent memory management, deploying per-VM memory encryption, and enforcing stricter privilege separation. We implement and validate Aster on two platforms: functional emulator that supports Android, and a performance prototype on an Arm board that captures microarchitectural aspects. Our in-depth evaluation of impact of Aster on protected virtual machines execution under stress benchmarks (CPU, system, IO) as well as representative applications (public key generation, One-Time-Password, isolated compilation) show the minimal runtime performance impact.