QoS of network transmissions can be done either at the vm level (basic) by setting a ks/sec limit for the virtual NIC or on the vSwitch-level (global policies). With the DVS you can select a rate limit (with units), and a burst size (with units). Traffic to all virtual NICs included in this policy level (e.g. you can create vm groups) is limited to the specified rate, with individual bursts limited to the specified number of packets. To prevent inheriting existing enforcement the QoS policy at the VM level should be disabled.

Background:
To limit the amount of outgoing data a VM can send per second, you can set an optional Quality of Service (QoS) value on VM virtual interfaces (VIFs). The setting lets you specify a maximum transmit rate for outgoing packets in kilobytes per second.
The QoS value limits the rate of transmission from the VM. As with many QoS approaches the QoS setting does not limit the amount of data the VM can receive. If such a limit is desired, Citrix recommends limiting the rate of incoming packets higher up in the network (for example, at the switch level).
Depending on networking stack configured in the pool, you can set the Quality of Service (QoS) value on VM virtual interfaces (VIFs) in one of two places-either a) on the vSwitch Controller or b) in XenServer (using theCLI or XenCenter).