答案：

builder = "hvm"
Explanation:
In Xen virtualization, guest domains can be created using either paravirtualization (PV) or hardware-assisted full virtualization, also known as HVM (Hardware Virtual Machine). According to official Xen documentation, the builder option in a Xen guest domain configuration file determines the type of virtualization used when creating a virtual machine.
To create a fully virtualized (HVM) guest, the configuration file must explicitly set the value of the builder option to "hvm":
builder = "hvm"
This instructs the Xen toolstack to use hardware-assisted virtualization features provided by the CPU, such as Intel VT-x or AMD-V, allowing unmodified guest operating systems to run. Fully virtualized guests behave like physical machines and can run operating systems that are unaware of Xen.
If the builder option is omitted or set to the default paravirtualized builder, Xen expects a PV-capable kernel, which requires guest operating system modifications. HVM guests, by contrast, rely on device emulation and hardware virtualization support.
Virtualization documentation clearly distinguishes between PV and HVM guests and identifies builder =
"hvm" as the required configuration for full virtualization. This setting is fundamental when deploying operating systems such as Windows or standard Linux distributions without Xen-specific kernels.
Therefore, the correct and documented value is builder = "hvm".