Nowadays, virtualization is a central element in data centers as it allows sharing server resources among multiple users across virtual machines (VM). These servers often follow a Non-Uniform Memory Access (NUMA) architecture, consisting of independent nodes with their own cache hierarchies and I/O controllers. In this work, we investigate the impact of such an architecture on network access. As network devices are typically connected to one particular NUMA node, this leads to a situation where device access on one node is faster than another. This phenomenon is called Non-Uniform I/O Access (NUIOA). This non-uniformity impacts the performance of I/O applications that are not executed on the correct NUMA node. In this paper, we are interested in NUIOA effects in virtualized environments. Our contribution in this work is twofold: 1) we thoroughly study the impact of NUIOA on application performance in VMs, and 2) we propose a resource allocation strategy for VMs that reduces the impact of NUIOA. We implemented our allocation strategy on the Xen hypervisor and carried out evaluations with well-known benchmarks to validate our strategy. The obtained results show that with our NUIOA allocation scheme, we can improve the performance of application in VMs by up to 20% compared to common allocation strategies.