When working with Xen virtualization, it's crucial to understand whether your guests are running in Hardware Virtual Machine (HVM) or Paravirtualization (PV) mode. This distinction affects performance characteristics, supported features, and potential troubleshooting approaches.

The most reliable way to determine the virtualization mode is by querying XenStore from within the guest OS:

xenstore-read vm-data/hvm

This command will return one of the following:

• "1" - Indicates HVM guest
• "0" - Indicates PV guest

For Linux guests, you can check loaded kernel modules:

lsmod | grep xen

PV guests typically show modules like:

xen_blkfront           28672  1
xen_netfront           28672  1

While HVM guests might show:

xen_platform_pci       16384  1

The system boot messages often contain virtualization information:

dmesg | grep -i xen

Look for lines like:

Xen HVM callback vector for event delivery is enabled

or

Booting paravirtualized kernel on Xen

If you have access to the host (Dom0), you can check guest properties:

xl list -l <domain-id> | grep type

This will output either "pvm" (PV) or "hvm" as the type.

Understanding the mode helps with troubleshooting:

• PV guests require PV-aware kernel and drivers
• HVM guests can run unmodified OS kernels
• Disk and network performance differs between modes
• Live migration capabilities vary

Here's a bash script to check multiple guests:

#!/bin/bash
for dom in $(xl list | awk 'NR>2 {print $1}'); do
    echo -n "$dom: "
    xl list -l $dom | grep -q '"type":"hvm"' && echo "HVM" || echo "PV"
done

This script lists all running domains with their virtualization type.

---

When working with Xen virtualization, guests can run in two primary modes: Hardware Virtual Machine (HVM) or Paravirtualization (PV). HVM guests require full hardware emulation, while PV guests use modified kernels for better performance through hypervisor calls.

The most reliable method is examining the guest's configuration file in /etc/xen/:

# Example for checking config
grep -E 'type=|kernel=' /etc/xen/vm-guest01.cfg

# Expected output for PV:
# kernel = "/boot/vmlinuz-2.6.18-xen"
# type = "pv"

# Expected output for HVM:
# type = "hvm"
# No kernel line present

For active guests, query XenStore data:

xenstore-read /local/domain/<domid>/image/ostype

# Returns "linux" for PV guests
# Returns "hvm" for HVM guests

Inside the guest OS, examine kernel modules and CPU flags:

# Method 1: Check loaded modules
lsmod | grep xen

# Method 2: Check CPU flags (HVM specific)
grep -E 'vmx|svm' /proc/cpuinfo

On newer Xen systems with xl toolstack:

xl list --long <domain-name> | grep type

# Sample output:
# (type pv)
# or
# (type hvm)

Some guests use PV drivers on HVM (PVHVM). Check for:

# In guest:
dmesg | grep -i xen
# Look for "Xen HVM" and "Xen PV drivers"

For your specific CentOS 5.2 environment:

#!/bin/bash
# Xen guest type detector for CentOS 5.2

if [ -f /etc/xen/xend-config.sxp ]; then
    CONFIG=$(grep -l "type.*hvm" /etc/xen/auto/*)
    if [ -n "$CONFIG" ]; then
        echo "HVM guest detected"
    else
        echo "PV guest detected"
    fi
else
    echo "Not a Xen guest or configuration not found"
fi