Trusted Execution Environments (TEE) are widely deployed, especially on smartphones. A recent trend in TEE development is the transition from vendor-controlled, single-purpose TEEs to open TEEs that host Trusted Applications (TAs) from multiple sources with independent tasks. This transition is expected to create a TA ecosystem needed for providing stronger and customized security to apps and OS running in the Rich Execution Environment (REE). However, the transition also poses two security challenges: enlarged attack surface resulted from the increased complexity of TAs and TEEs; the lack of trust (or isolation) among TAs and the TEE. In this paper, we first present a comprehensive analysis on the recent CVEs related to TEE and the need of multiple TEE scheme. We then propose TEEv, a TEE virtualization architecture that supports multiple isolated, restricted TEE instances (i.e., vTEEs) running concurrently. Relying on a tiny hypervisor (we call it TEE-visor), TEEv allows TEE instances from different vendors to run in isolation on the same smartphone and to host their own TAs. Therefore, a compromised vTEE cannot affect its peers or REE; TAs no longer have to run in untrusted/unsuitable TEEs. We have implemented TEEv on a development board and a real smartphone, which runs multiple commercial TEE instances